A Prince George's County man described by law enforcement officials as a major heroin trafficker in Washington pleaded guilty in U.S. District Court yesterday to conspiracy to distribute heroin and to tax evasion.

Assistant U.S. Attorney Daniel J. Bernstein told the court yesterday that as part of a plea bargain agreement with the government, Willie (Black Willie) Bullock, 37, will also plead guilty to distribution of heroin in Maryland and conspiracy to distribute heroin in New York.

Bullock, who is now being held in D.C. Jail on a $200,000 bond, faces a total of 50 years in prison in connection with the three cases, Bernstein said.

Bernstein also told Judge Thomas A. Flannery yesterday that Internal Revenue Service agents have seized numerous bank accounts connected with Bullock as well as the assets of U.S. Racing Inc. in Tuxedo, a corporation owned by Bullock which enters cars in drag racing competitions throughout the country. Liens have also been placed on five pieces of property Bullock owns, Bernstein told the court.

The seizures were made to cover taxes owed from 1976 to 1979, Bernstein said. Bullock pleaded guilty yesterday to evading taxes in 1979 by underreporting his income by $90,000. At the time of his arrest, law enforcement officials said, Bullock owned several luxury cars and lived in a custom-built home in rural Prince George's County.

Bullock was arrested last August after a grand jury indictment on charges that he operated a heroin distribution ring with sales of more than $1 million a year. That operation allegedly focused on some of Washington's most notorious drug dealing centers, including the 900 block of O Street NW, 14th Street and Wallach Place NW and the 1300 block of T Street NW.

Bullock was released from custody after posting a $5,000 bond. He was taken back into custody in December by federal agents in Brooklyn after he allegedly negotiated for the sale of a large quantity of high-grade heroin there. According to federal agents, Bullock had also sold heroin to an undercover agent in Maryland in September.

In exchange for Bullock's guilty plea, the government has agreed to drop additional drug and tax charges against him. No date has been set for sentencing on the charges to which he pleaded guilty.