By JUAN FORERO with TIM WEINER

SAN ROQUE, Colombia — Colombia and Mexico have become the dominant suppliers of heroin to the United States, supplanting Asia, in a trend that experts
and the authorities fear could offset American-backed successes in a campaign against drugs that has focused mostly on cocaine.

Here in the lush, nearly impassable mountains of Tolima Province, rebels of Colombia's largest guerrilla group stand watch near muddy footpaths leading to opium
farms that experts say help produce upward of 80 percent of the heroin that reaches American streets.

From Maine to California, law enforcement authorities report small-scale epidemics and a rising rate of overdoses from a dangerously potent and cheap form of
heroin. While total heroin use in the United States has not risen significantly, the drug is appealing to new, middle-class users because it can be smoked or snorted,
rather than injected.

After steadily expanding its market in recent years, white Colombian heroin now dominates east of the Mississippi; brown Mexican heroin rules to the west. The
pattern signals an alliance between Colombian and Mexican traffickers, one American official said.

Evidence of the shift from coca to opium poppy can be found across Latin America, which still produces just a fraction of the heroin made worldwide — mostly in
places like Afghanistan, Myanmar and Pakistan — but the vast majority reaching American users, the authorities say.

New opium fields have been discovered in Peru, which until recently had made great strides against coca. Strands of poppies are also increasingly being spotted along the Venezuelan border, according to Colombian government officials.

The shift, experts and American authorities fear, could present a new challenge to aggressive American-financed efforts to fight the illegal drug trade in Colombia with aerial fumigation of coca, a lowland crop used to make cocaine. Heroin may provide a potentially important new source of financing for the leftist rebels and
right-wing paramilitary groups that depend on drug money to wage war. Unlike coca, the plant used to make cocaine, opium poppies are grown high in
cloud-shrouded mountains and in ever smaller and scattered plots, they say.

When crop-dusters arrive, officials said, traffickers or rebels often open fire on them. Opium traffickers in Mexico have already shot down three army helicopters this year in the southern state of Guerrero. Here in rugged southern Colombia, a one-acre plot belongs to Fernay Lugo, rail thin and agile, who works, razor in hand,
slicing open the pods of his blossoming poppies to collect the milky gum that is refined into heroin.

He explained how — day after day, bit by bit, in mountains 7,000 feet up — he tries to accumulate a few pounds, enough to sell for the kind of profits his slumping
coffee plants could never fetch.

He does not ponder who his buyers are, the shadowy men who meet him at a distant roadside, or their ultimate customers. "When we harvest and sell, we do not
even think where it goes," said Mr. Lugo, 29, the father of two girls. Though farms here, including Mr. Lugo's own, have been hit by crop-dusters in the past, he
seemed to show little concern.

The skies are rarely clear over southern Tolima, the cloud cover often so heavy that fumigation planes cannot come in. Farmers also disperse their poppy crops, Mr.
Lugo said, to make them harder to identify by satellite and reconnaissance aircraft.

In this region, some of the greatest inroads in eliminating poppy plots have been made not through aerial spraying but from programs that pay farmers to eradicate the
crop and switch to legitimate ones. Still, it is not hard to find the brilliant lavender and red flowers of the mature opium plants that so sharply contrast against the
monotonous drab green hues of the legal crops that peasants also grow.

Blanca Ruby Pérez, 39, said she and her family lived by poppies, which can be harvested twice a year and bring far more money than blackberries, corn, beans and
lettuce. "It is much easier to grow than the other crops," she said, carefully tiptoeing around the small, green leaves. "Look, we have put no fertilizer on it, and look
how pretty it is."

Once it is processed into heroin and smuggled to the United States, its effects are anything but pretty, law enforcement officials say. The heroin is "way better, in terms of purity, both Mexican and Colombian," than in years past, one official said.

With improving purity and lower costs has come increasing use. The number of hard-core users in the United States rose to nearly a million last year, from 600,000 a decade ago, said the Drug Enforcement Administration. In New York State, 32,000 people were admitted to state-licensed drug treatment centers for heroin
addictions last year, up from 29,000 in 1997. The government's National Household Survey on Drug Abuse also determined that the number of 18- to 25-year-olds
who had used heroin in the last month rose to 67,000 in 2001 from 26,000 in 2000, which some experts say shows more young people are finding the new,
high-power heroin more palatable.

Many new heroin users are turning up in unexpected places, not just the "shooting galleries" in tough urban neighborhoods where addicts found their fix in years past.

In Portland, Me., a city of 64,000, the number of people dying from overdoses rose to 28 last year, most of them heroin users, Detective Sgt. Scott Pelletier said in a telephone interview. In 2001, 16 died.

The police in Portland say heroin has become readily available, with the price of single-dose bags as little as $15. They were once sold for at least $35, and
sometimes up to $50 in the late 1990's. "It's our No. 1 priority, as far as drug investigations go," said Detective Pelletier, who oversees the city's narcotics unit. "It's
everywhere. We're just beginning to see, over the last year, what we call raw heroin, where people buy it by the gram, rather than in a single dose."

Soaring seizures of Colombian heroin in Panama and Nicaragua point to the growing importance of those countries as corridors in the northbound drug pipeline, in
which smugglers use every method available to move their product, from welding kilos into the drive shafts of cars to soaking the powder into coat linings or
swallowing it in condoms.

Couriers carrying Colombian heroin commonly hopscotch Central American airports to Mexico. From there, they drive or carry both Colombian and Mexican heroin across the border into the United States, usually less than 10 kilograms at a time, an amount that can be worth as much as $1.5 million wholesale.

The heroin may also be brought to the United States by air and sea across the Gulf of Mexico and the Caribbean. The transportation and sales are not always
controlled by the powerful kingpins and established drug gangs, but by what one official called "mom-and-pop operations that are hard to penetrate."

Law enforcement officials say that in the northeastern United States, a Colombian system that for years ran cocaine is now increasingly reliant on heroin, which is
easier to smuggle and brings much more profit.

"The Colombian traffickers have had their distribution networks in place," said Bridget Brennan, the special narcotics prosecutor for New York City. "They've already been dealing in cocaine. It was a market that was ripe for the picking."

This month, officials from the United States, Colombia and Mexico say they will meet to seek new ways to combat the heroin trade. But the same factors that make
heroin poppies hard to eradicate also make it hard even to determine exactly how much exists, or exactly how much is winding up in the United States.

"It is difficult to detect, and difficult to fumigate," explained Klaus Nyholm, chief representative in Colombia of the United Nations Drug Control Program. "So, to sum it up, we don't know."

Optimistic reports from the White House drug control office estimate that the size of opium fields was reduced last year by 25 percent in Colombia and 40 percent in
Mexico. But using new research techniques, the same American drug enforcement analysts say the amount of Colombian heroin produced last year is three times the
4.3 metric tons previously assumed.

Drug enforcement officials agree now that Colombian and Mexican supplies of heroin "pretty much satisfy the market in the United States," as one law- enforcement
official put it. Despite the unknowns, officials in all three nations cite the same evidence for the new dominance of Colombian and Mexican heroin: rising seizures,
higher purity, falling prices.

Seizures of heroin reported by the United States customs officials in 2002 totaled 5,598 pounds, an estimated 80 percent of it from Colombia, up from 3,521 pounds the previous year.

As for refining the drug, both American and Mexican officials say they suspect Colombians have taught Mexicans new techniques for creating purer and more valuable heroin powder from opium gum.

"We see more sophisticated processes of refinement and deeper knowledge in the chemistry of production" in Mexican heroin, said José Luis Santiago Vasconcelos,
Mexico's top organized-crime fighter. "It's very similar to the Colombians' techniques."

The price of heroin on the street, meanwhile, has dropped sharply, from as much as $220,000 a kilogram for Asian heroin a decade ago to as little as $60,000 now,
said law enforcement officials in New York and elsewhere.

"It's outrageously low compared to what it used to be," Anthony P. Placido, the agent in charge of the D.E.A. in New York, said in a telephone interview.

In December, Congress held a hearing in which a host of drug experts and law enforcement officials warned of the threat of Latin American heroin. But conservatives in Congress are complaining that little has been done to eradicate opium poppies.

"A miscalculation in our strategy was to obviously ignore the poppy cultivation, and we paid for it with an increase in supply," Representative John L. Mica,
Republican of Florida and a member of the House subcommittee on drug policy issues, said by telephone from Washington.

American antidrug officials who work in Colombia, however, defend their spraying as extensive, and say it will be more aggressive this year. In 2000, pilots sprayed
about 22,000 acres of opium poppies in Colombia, according to Congressional testimony in Washington. In the next two years, the number of acres sprayed fell, with 4,900 acres eradicated in 2001 and 8,100 last year.

One leading counterdrug official said in an interview in the Colombian capital, "We essentially sprayed all we could find."