State officials today announced that fertile Mediterranean fruit flies have been found in Los Angeles County, a 300-mile leap from the state's quarantine zone.

The discovery of nine Medfly larvae in a peach and eight flies in traps in the eastern Los Angeles community of Baldwin Park was a severe blow to the anti-Medfly campaign, and led to the quarantining of 81 square miles of Los Angeles County. A helicopter rushed to Los Angeles tonight began spraying a nine-square-mile area with the pesticide malathion.

"We're very disappointed," said Dick Thompson, a spokeman for the anti-Medfly campaign, headquartered in Los Gatos, Calif. If the flies came from the six-county quarantine zone in northern California, as offiicals said they strongly suspect, it is the most dramatic sign yet of the state's failure to stop motorists from carryingf infested fruits and vegetables out of the quarantine zone.

The Japanese government, unimpressed with U.S. claims that California fruits from outside the northern Californiat quarantine zone are safe, announced early today that it will insist on fumigation or some other treatment of all Medfly-susceptible produce from the state.

State and federal officials said they fear that such bans might spread to other countries and other states if the insect is not brought under control soon. a spokesman for the Agriculture Department said today that there are no plans, however, to enforce a federal quarantine on all of California, which provides about half the nation's fruits and vegetables.

California's agriculture industry amounts to about $16 billion a year. Representatives of growers in the rich Central Valley say they are equipped to fumigate only about 5 percent of their crop.

Japan, the state's leading foreign customer for fresh produce, imported about $168 million in California produce last year, a State Department spokesman said. It is uncertain how much of that business may be lost. Growers' representatives such as the Sunkist group said they hoped to be able to fumigate a substantial part of the Japan-bound crop if there are not any more major demands for fumigation.

Los Angeles County Agriculture Commissioner Paul Engler, in announcing the county's plans to stop the new infestation, said, "With a volume of traffic to Los Angeles, it was just a matter of time before we had the pest in southern California."

State officials acknowledge that unless they can seal off the more than 3,000 quarantined square miles of northern California, about 2 percent of the state, or persuade motorists to cooperate, they face months of stamping out small outbreaks of the insects in widely scattered locations.

They remain confident, however, that they can catch any new infestations early because they have placed so many new Medfly traps around the state.

The small fruit fly lays eggs inside more than 200 different varieties of fruits and vegetables. The eggs hatch into larvae, or maggots, which eat the fruit and render it unmarketable. Malathion, mixed with a syrup that attracts the insects and dropped over an infested area, kills the creature by paralyzing its nervous system.

The first fertile Medflies in Los Angeles to be found Tuesday were in a trap in a pineapple-guava tree behind the home of Dorothy Davies. Davies said a woman from the county agriculture department made her weekly check of the trap Tuesday morning "and said she found flies that looked suspicious."

The woman took the trap, and other department workers collected samples of peaches, avocados, guavas and grapes to be sent to a laboratory in Sacramento for analysis. State officials said three of the flies collected Tuesday were too dried to analyze, but two were found to be fertile, and the presence of larvae in one peach confirmed the infestation. Engler said three more flies were found today.

About 300 young members of the California Conservation Corps went through the neighborhoods today informing residents of the spraying. State officials said they will spray the area at seven- to 10- day intervals for six to eight weeks. "What we find determines what we will do," Engler said.

State crews also have flooded the area with new Medfly traps, about 100 for every square mile, in order to quicken response time. Southern California has had at least two previous Medfly outbreaks, in 1975 in Marina Del Rey and last year in the San Fernando Valley, but this is the first time aerial spraying has been used there against the pest.

In previous outbreaks officials sprayed malathion on the ground and released sterile Medflies to interrupt the insect's breeding cycle. Previous infestations were blamed on fruits brought in from Hawaii or possibly Central America, which officials said might also have been the source of the latest outbreak.