Men in black SWAT suits, with pistols on their hips and combat knives sheathed to their thighs, stand guard over vast fields of blue agave plants, prized for the juice that produces tequila. Around the clock they patrol the dirt paths that crisscross endless acres of the cactus-like plants, five-foot-tall starbursts that tint the valley floor smoky-blue in the namesake capital of Mexico's national drink.

Until four months ago, the only security in these hot, sleepy fields 300 miles northwest of Mexico City was Roberto Castaneda Flores, an old mustachioed cowboy nudging his ancient horse along with jangling spurs. But as global demand for suddenly chic tequila booms, and as farmers seek to make up for the ravages of a disease that killed millions of plants a couple of years ago, the supply of blue agave plants is dwindling. That has made them increasingly expensive--their market price has gone up by more than 10 times in a year--and increasingly targeted by thieves.

In response, the wealthy distillers in this valley where almost all tequila is produced have deployed a private army to protect their agave plants, which are as important to Mexican identity, and suddenly almost as valuable, as the masterpieces of Diego Rivera or Frida Kahlo.

"When a person steals agave, he offends the people of Mexico," said Fernando Flores Zuniga, head of security for Jose Cuervo, the world's leading tequila producer. "They are stealing our history, part of our cultural identity."

Cuervo has sent 125 security guards into its agave fields since April, when it created a security department for the first time in its 205-year history. Even with the new guard force, thieves this week hacked down 120 agaves and stole their juice-rich cores, which weigh up to 150 pounds each and look like monstrous pineapples. That stolen load was worth more than $8,000 at the distillery, a massive sum in a region where most laborers don't earn that much in a year.

Thefts since the tequila boom began, ranging from two agaves to 70 tons of cores worth nearly $70,000, have unsettled Tequila, where most of the streets are cobblestone and most of the buildings belong to Cuervo or Sauza, Mexico's other major tequila producer. Overwhelmed local police have happily accepted assistance from the rich tequila companies. Flores said Cuervo has supplied the Tequila police force with two pick-up trucks, radios, flashlights, tents, raincoats, food and other supplies to help keep an eye on the agave fields.

Tequila is more than a drink in Mexico. It is a national passion shared by rich and poor, Indians and those of European ancestry, and which traces its origins to Aztec priests who discovered the agave's potent qualities.

Like Bordeaux or Champagne, Tequila is more than a name. It is the proprietary product, and the pride of, a distinct region. In Mexico's case, that region locates its center in this flush company town of 35,000 people, which announces itself to visitors with a billboard: "Welcome to Tequila, Population: 100 Percent Agave."

The unusual rise in crime here, where almost everyone depends on tequila for his livelihood, is a direct result of the phenomenal global success of tequila. In the past five years, tequila's image has undergone a remarkable image makeover, evolving from frat-party booze to a chic sipping drink selling for top-shelf prices in fancy bars from San Francisco to London.

"People are more interested in tequila now; they don't want to just do shooters," said Dan Mesches, a partner in the Red Sage restaurant at 14th and F streets in Washington, which carries a selection of 46 tequilas with prices ranging up to $16 a shot for a Porfidio Anejo Cactus with its "smooth peppery finish."

"Just five years ago, tequila companies were begging us to carry their product," Mesches said in a telephone interview. "But now, as people become more affluent, they are seeing tequila not just as a rot-gut liquor, but as something you can use as an after-dinner drink in a snifter."

Credit marketing. The major names in tequila, including Cuervo, are now partly owned by global liquor conglomerates, such as Seagram Co. and United Distillers of Britain, which have used such gimmicks as crystal decanters, leather gift bags and "limited edition" bottlings selling for hundreds of dollars to entice wealthy, status-conscious American and European sippers.

Several "special edition" tequilas marketed as "millennium" editions earlier this year sold for as much as $750 a bottle. The most expensive labels are those made from 100 percent agave juice; the vast majority of tequilas are 51 percent agave, the minimum required by Mexican law, and 49 percent distilled cane sugar.

Tequila was once considered the most macho of macho drinks. In addition to Mexican ranch-hands, it was also favored by the very embodiment of macho, John Wayne. In a 1977 letter to Sauza, which is proudly reprinted in a glossy coffee table book here, Wayne noted that "Your very special product has become as necessary in our household as air and water."

But now the rough charms of tequila are even being marketed specifically to Mexican women, who sip the stuff straight over business lunches in Mexico City. And margaritas, made with tequila, are among the most popular drinks in the United States with both sexes.

In response to the recent boom, tequila production has almost quadrupled in the past 10 years, from less than 50 million liters in 1990 to more than 190 million last year. Of that, 97 million liters were exported, 80 percent of it to the United States. A decade ago, there were 35 brands of tequila in Mexico; now there are about 580. And there has been a corresponding hike in agave harvesting.

The problem is that agave plants take up to 12 years to mature. And the plantings of a dozen years ago never anticipated that this once humble crop would someday be the hottest thing from Adams Morgan to SoHo. That, combined with the disease that destroyed millions of agave plants a couple of years ago, has led to a severe shortage of agave, the only plant under Mexican law that can be used to produce tequila. The Mexican government says there are a third fewer agave plants growing now than there were three years ago.

The dramatically increasing prices for raw agave have been passed on to consumers. Mesches, of the Red Sage, said prices for most tequilas have more than doubled in the last year.

In the Tequila valley, Jose Luis Gama, 53, is delighted to hear that customers at trendy American restaurants enjoy the fruits of fields where he now works with his son, and where his grandfather and father worked before him. For most of his life he has been out here, hacking the long, spiky leaves off agave plants with a coa, which looks remarkably like the long-handled paddles used to remove pizzas from hot ovens, except that the coa is sharp as a razor.

Gama said the only thing new is the armed guards watching him. But, he said, smiling under his sweat-stained straw hat, "I'm glad they're here."