HREE RIVERS, Calif. ÷ Wearing camouflage and armed with machetes, a dozen police officers waited in the gathering heat of morning for a helicopter that was to drop them into a marijuana farm, in the hills two miles distant.

Rattlesnakes, poison oak and bee stings would be the least of their concerns, the officers were warned before the raid: They might be welcomed with gunfire.

"You've got to take care of yourselves," Sgt. Richard Matthews, a SWAT team leader for the Tulare County Sheriff's Department, told the officers in a briefing near here, at the western boundary of Sequoia National Park.

Almost every day through the marijuana harvest season, which recently ended, federal agents and the local police descended on the increasingly large pot farms in California's national forests, looking for the growers and their possible connections to Mexican drug traffickers.

"The Mexican cartels have taken over the industry, and when they do something, they don't do it in small amounts," said Sonya Arriaga Barna, operations commander for a California Department of Justice task force, Campaign Against Marijuana Planting, or CAMP, which coordinates some of the law enforcement teams that conduct pot raids around the state. The Drug Enforcement Agency says that 935,680 plants, worth about $3.7 billion on the street, were seized in California this summer and early fall, the most ever.

Almost half of the plants seized in California were uprooted from national forests. Seizures in California ÷ where Mexican drug organizations appear to have concentrated their cultivation and the leader, by far, in the national marijuana stakes ÷ have more than doubled in the last four years. Of the top 10 forests around the country where pot was seized last year, six were here.

About 720,000 plants were eradicated nationwide from Forest Service lands in 2001, more than twice the 1997 figure. This year's numbers are still being compiled.

Law enforcement officials in the Sequoia National Forest and in others across the West say marijuana groves are an increasingly frequent and often undetected feature of the vast terrain. Far from places usually visited by tourists, the plantations thrive.

This year's crop continued a trend of huge growth in the size and number of pot farms run by Mexican traffickers, officials say. Facing tighter border controls and more effective policing in Mexico, they have switched from smuggling their crop to planting it on United States soil.

"Why take the risk of smuggling marijuana over the border when you can come here to grow it?" asked Sgt. Marsh Carter, who until recently led the SWAT team for the Tulare County Sheriff's Department, which covers much of the Sequoia forest.

Laura Mark, assistant special agent in charge of the Forest Service's drug eradication effort in the West, said: "Ninety to 95 percent of the marijuana in California is now grown by Mexican nationals who work for the drug cartels. The dynamics of it have changed. This isn't about hippies anymore."

Mike Delaney, who oversees marijuana eradication for the Drug Enforcement Agency in Northern California, said it appeared that "command and control" of at least some of the plantations originated with "organizations based in Mexico."

"The majority of these growers are armed, and that poses a threat if someone is hiking or camping in the wilderness," Mr. Delaney said. While no campers or hikers are known to have been harmed, several law enforcement officers have been injured in shootouts. At least two suspected growers have been fatally shot in raids.

No growers were in the plantation near Three Rivers, 80 miles southeast of Fresno, when the 12 law enforcement officers flew in. The growers may have been scared away by a raid on a nearby plantation a week earlier. Still, 3,846 plants were hauled out in nets by the helicopter and destined for burning.

Planting on federal land leaves no property owners to prosecute; the traffickers cannot be identified unless they are caught visiting their crops.

In June, nine men with ties to what prosecutors called the Maga–a cartel pleaded guilty in federal court in Fresno to a variety of drug charges. The men were arrested in October 2000, with more than 30 others, after they were connected to more than 100,000 marijuana plants in the Sequoia National Forest. The authorities said the cartels were responsible for about 80 percent of the pot grown in the Sierra Nevada.

Sergeant Carter, the former SWAT team leader, said detectives had identified four other organizations in the area. "Ninety-nine percent of the people we arrest or investigate are from Mexico," he said.

A report by the law enforcement and investigations unit of the Forest Service said that organizations in Mexico were supplying workers to tend marijuana on Forest Service lands in California and in Arkansas, Idaho, Oregon, Utah and Washington.

Investigators in the Maga–a case said cartel leaders brought in illegal workers from the Mexican states of Michoac‡n and Jalisco. Others were recruited from California street corners, where they gather in search of day jobs. The recruiters often tell them they will be working in "agriculture" or "cutting wood." The men are generally taken to remote places, given equipment, seeds, fertilizer and tents and sometimes weapons ÷ and told to stay there, sometimes for months. They might earn $5,000 or $6,000.

"A lot of these guys don't want to be there," said Doug Babb, a recently retired Tulare County deputy. "We've talked to guys who say they're basically forced into slave labor."

The garden tenders are often violent. "These guys shoot people," said Ms. Mark, the Forest Service official. "They do not distinguish between police, hunters or campers. As far as they're concerned, everyone is a pot thief."

Officers investigating pot workers' camps have found elaborate treehouses, makeshift showers and, invariably, trash and pesticides, much of which ends up in streams and creeks. The crops are often surrounded by barbed wire, and sometimes irrigated with water from nearby streams.

Wildfires this summer in the Sequoia forest destroyed several pot farms and flushed many of the tenders from hiding.

"When the big burn happened, a lot of guys came out of the hills," said Jan Barnes, a resident of Springville, 20 miles south of here. "They looked like they were starving, and they were filthy."

People are getting accustomed to the pot farms. They are still talking about the smoke that wafted over town two summers ago, when plants were burned at the county dump south of Springville.

Firefighters for the California Department of Forestry and Fire Protection were exempted from random drug tests for three months because of their exposure to the marijuana cloud.

"The town loved it," Steve Kelley, a California Highway Patrol trooper based in Springville, said, not entirely seriously. "They asked for more."