A New York art importer whose goods were seized in a crackdown on illegal art imports pleaded guilty yesterday to undervaluing $288,000 worth of pre-Columbian artifacts taken from ancient Peruvian burial grounds.

Customs agents had seized the goods at Dulles International Airport last year as part of an effort to end what some underdeveloped countries contend is a plundering of their national treasures.

Art importer David Bernstein ended a criminal investigation into his business by pleading guilty in Alexandria to a misdeameanor charge of filing a false customs declaration and by agreeing to return the seized items to Peru.

Prosecutors said that when the 35-year-old art dealer arrived at Dulles last January from South America he said his four suitcases held 66 Peruvian artifacts worth $1,785. Skeptical customs agents opened the bags and found precious pre-Columbian textiles, gold alloy death masks, a rare feathered poncho, a ceramic pot from 800 B.C.--a total of 158 pieces. All the items had been removed from ancient Peruvian graveyards and were valued at $288,000, investigators said yesterday.

In addition to those items, which are being held at customs offices in Reston under carefully controlled heat and temperature conditions because of their antiquity, federal agents armed with a search warrant later seized at least 160 other artifacts valued at $1.4 million at Bernstein's home in Manhattan.

Under terms of the plea arrangement, Bernstein has agreed to return all artifacts that he cannot prove were purchased outside of Peru.

A customs official who asked not to be identified said yesterday the value of such items has increased in the last year since the federal crackdown began.

Customs officials and attorneys for Peru said yesterday's plea was the third in recent weeks in which American importers have agreed to return Peruvian ethnological art treasures. One case in Miami involves items valued at $30,000, they said. An American living in Argentina also has agreed in U.S. Magistrate's Court in Alexandria to turn back a half-dozen artifacts imported into the United States from Peru.

Washington attorneys Frederic J. Truslow and Eugene Propper, who represent the Peruvian government, said the artifacts involved in all three cases soon will be turned over to the Peruvian Embassy for shipment back to South America.

"The Bernstein case is significant for the number of pieces and the importance of the person in terms of the amount of stuff he's taken out of Peru over the years," Truslow said yesterday.

Propper said the Peruvian government has three goals--"the return of the property, deterrence and punishment"--in its campaign to recover artifacts that, under a national patrimony law, it considers stolen when taken out of the country.

In pleading guilty to a misdemeanor, which carries a maximum penalty of one year in prison and a $1,000 fine, Bernstein agreed yesterday to cooperate with federal prosecutors in their continuing investigation of pre-Columbian art imports.

That includes disclosing the identities of art suppliers and buyers, opening his files to the government and testifying before federal grand juries in other cases if requested, according to the written agreement submitted to U.S. District Judge Richard L. Williams.

Although the United States has bilateral agreements with Peru and Mexico to aid in efforts to return artifacts and prevent future illegal art imports, a 1972 United Nations treaty governing imports from many other foreign countries has yet to be ratified by Congress.

Judge Williams set sentencing for Bernstein for Feb. 19 in Alexandria.