Swarms of bee hunters are combing a 400-square-mile area in the oil fields of central California, trying to determine whether Africanized honeybees -- popularly known as killer bees -- had spread before oil workers discovered an underground colony late last month.

That colony -- destroyed with an aerosol fumigant -- was the first documented sighting of the feared bees in the United States other than isolated insects found on ships that had just arrived from Latin America.

Africanized bees are descendants of colonies imported from Africa to Brazil in the 1950s that escaped to breed with local varieties. The hybrid bees, which are much more prone than other honeybees to sting when disturbed, have been spreading northward ever since. They now occupy all of South America and have moved up the Central American peninsula as far as Honduras.

Though more likely to sting, the hybrid bees have venom no more toxic than that of the conventional European honeybee common to the United States. There is no evidence that any more people have died from their stings than from those of European bees.

Beekeepers fear the Africanized bee because it breeds readily with the preferred European honeybees, producing a hybrid with all the traits of the Africanized strain. Africanized bees produce less honey than the European varieties, and beekeepers consider them more dangerous to keep.

In most countries where the bees have spread, commercial honey production has dropped, mainly because people get out of the business. Venezuela, for example, produced 578 tons of honey before the arrival of the Africanized bee but less than 100 tons after it took over the country.

Bee specialists from the U.S. Department of Agriculture had been expecting the Africanized bees to reach the United States around 1988 to 1990 and, if nothing were done to stop them, spread throughout the country.

Discovery of the wild colony in California, in a semidesert area about 100 miles due north of Los Angeles, has led to speculation that the bees leapfrogged over Mexico by stowing away in oil-drilling equipment carried by freighter from South America to California.

The colony was found last month when an oil worker saw bees, apparently disturbed by vibrations from the heavy equipment he drove, fly out of an abandoned fox burrow and attack a rabbit.

The worker poured hot asphalt into the hole but the next day found that the insects had tunneled around the asphalt to get out. He summoned agriculture officials, who fumigated the burrow.

Although first reports suggested the colony was fairly new, just off the boat, there is evidence that it had been in California for at least a few months and perhaps since last year -- long enough to have spread.

"They dug down a lot deeper and found it was a much bigger colony than they first thought, and they found the bees had already started a comb," said Allen Sylvester, an expert on Africanized bees, at the U.S. Agricultural Research Service station in Baton Rouge, La.

California agriculture department workers have been sifting through the hive, which reached about four feet underground, searching for a queen bee among the dead workers and drones. So far, no queen has been found. Officials fear she may have fled before the fumigation and may still be able to establish a colony elsewhere or take over a European bee colony lacking a healthy queen.

Sylvester said there are probably other colonies of Africanized bees in the region but expressed confidence that they can be found and killed before spreading farther.

"I think we're in for a situation like the medfly," Sylvester said, referring to the infestation of the Mediterranean fruit fly that was found in California and eradicated with quarantines, pesticides and destruction of potentially infested fruit.

California agriculture officials placed an embargo on movement of hives out of the area until inspectors can verify that they contain none of the invading bees. Africanized bees look like a slightly smaller version of the European bee but are most readily identified by their behavior when disturbed.

The Associated Press reported that federal, state and county workers checked 36 of the 110 commercial beekeeping operations Friday near the oil field where the Africanized bees were found. They contained only domestic bees, according to the state's Department of Food and Agriculture.

Sylvester said it may be necessary to prohibit bringing hives into the area. Beekeepers often earn money by trucking their hives hundreds of miles to areas where growers want to ensure good pollination of their crops.

Sylvester was optimistic that the current infestation can be stopped but held out no hope that the wave spreading up from Central America can be prevented from taking over the United States.

"Eventually, we're going to be living with the Africanized bee, whether we like it or not," he said.

California officials are not as resigned to the invasion of the Africanized bees. "Those of us in positions of responsibility for protecting the health and safety of Californians will not accept this option," said Isi Siddiqui, assistant director of the state agriculture department.

He said that when the bees reach the Mexican border California will be ready with traps and pesticide sprays to stop their advance.