The longstanding U.S. ban on Mexican avocado imports is unlikely to be lifted in time for Mexico's winter crop to be shipped next month, meaning that imports probably won't be allowed before November 1996 at the earliest, U.S. Agriculture Department officials said this week.

The department had envisioned imports beginning this November when it proposed last July to lift the 80-year-old ban on imports into the continental United States.But vocal opposition from California growers, who produce virtually the entire U.S. crop, has slowed the approval process, officials said.

Officials of the department's office of Animal and Plant Health Inspection Service (Aphis) now say the time required to evaluate more than 1,000 comments received during the 60-day comment period that ended last week will probably push a final decision into next year, possibly past next February when Mexico's upcoming shipping season ends.

The Aphis proposal would allow avocados from Mexico's Michoacan region to be shipped to 19 Northeastern states from November through February, the off-season for U.S. growers.

The delay in implementing the order is at least a temporary victory for California's $251 million avocado industry, which faces potentially ruinous competition from Mexican growers in the key Northeast market. The price of U.S.-grown avocados is two to three times the price that Mexican avocados currently sell for in Canada, for example, where imports have been allowed for several years.

Lifting the ban would also hurt because U.S. growers are heavily reliant on the U.S. market, having been slower than growers of many other agricultural commodities to cultivate export markets.

''All we had asked for was a fair review of the information and the data, and that is good if they are doing that," said Cherrie Watte, director of national affairs at the California Farm Bureau.

Avocados are by far the most contentious of the roughly 150 crops that Aphis is now considering for import permits.

The issue is shaping up as an early test of Agriculture Secretary Dan Glickman's policy of judging agricultural import restrictions by sound scientific standards, as mandated under rules of the World Trade Organization.

Mr. Glickman has been vocal in pressing foreign countries to dismantle similar import barriers that may be designed primarily to protect local farmers.

Observers say that lifting the avocado ban, which Aphis' scientists say would pose no threat to the U.S. crop, would show Mr. Glickman's resolve in applying that standard with equal force to the U.S. market.

If Aphis decides after reviewing the comments that the ban can be lifted without endangering the U.S. crop, the final decision would be the agriculture secretary's, department officials said.

The proposal sparked an outcry from U.S. avocado growers, who said it would fail to protect the U.S. crop from seven avocado-attacking pests known to exist in Mexico.

At an unprecedented five public hearings held throughout the country in August and September, growers told an Aphis scientific panel that Mexico has not done nearly enough to eradicate the pests in its growing region.

They said the elaborate procedures Aphis plans to mandate as a condition of lifting the import ban, including rigorous pruning of dead branches where pests thrive, would still not stop the pests from crossing the border.

Growers also said there would be no effective way to contain avocados in the Northeast, noting that the limited number of avocados currently allowed into Alaska often wind up in Washington and other states in the Pacific Northwest.

The Northeast was chosen as the import area because winter temperatures there would theoretically kill off any seed weevils and other insects from Mexico known to attack avocado trees.