U.S. Attorney Joseph E. diGenova, who has directed a series of highly publicized investigations of alleged city government corruption, is negotiating to join a Washington law firm, and an agreement could be made final as early as today, sources at the law firm said last night.

DiGenova, whose four-year term as U.S. attorney expired Dec. 3, was in Paris on business and could not be reached for comment. A spokesman for diGenova's office said he would not comment.

The law firm -- Bishop, Cook, Purcell & Reynolds -- has 85 lawyers here and is involved in the full range of federal practice, including lobbying. Sources at the firm said they were "very excited" that diGenova might join them. DiGenova had been known to be exploring possible jobs that would use his extensive legislative experience.

DiGenova, who probably will remain as U.S. attorney for about six weeks, said earlier that his leaving would not affect investigations, including the probes of D.C. contracting practices and skimming of drugs and money by vice officers in the 4th Police District.

"Things are well enough along that I feel comfortable about leaving, knowing that all investigations that are outstanding will be completed," diGenova said last month.

Previous investigations by diGenova's office have led to the convictions of 12 former D.C. government officials, including deputy mayors Ivanhoe Donaldson and Alphonse G. Hill, both of whom are now in prison.

Mayor Marion Barry has accused diGenova of trying to run him out of office, and supporters of the mayor have said the Reagan administration is conducting a systematic attack on black officials.

At one point, diGenova characterized the city government as an example of "raw corruption."

Although investigations of city officials have intermittently dominated the headlines during diGenova's term, he has also directed the prosecution of convicted spy Jonathan J. Pollard and former Reagan administration official Paul Thayer for insider trading. His office also has conducted a number of major drug prosecutions.

Under a recently passed law, Attorney General Edwin Meese III could appoint an interim U.S. attorney for 120 days while a permanent replacement for diGenova is sought. Among those who might be named for the interim are Timothy J. Reardon III, diGenova's principal assistant, and David F. Geneson, a former Justice Department lawyer who is now an assistant U.S. attorney.

DiGenova, a graduate of Georgetown University law school, joined the U.S. attorney's office in early 1982 as principal assistant to Stanley S. Harris, who is now a U.S. district judge.

DiGenova was appointed U.S. attorney and was sworn in on Dec. 3, 1983. DiGenova previously served as an assistant U.S. attorney, prosecuting cases in D.C. Superior Court, in the 1970s before joining the staff of the Senate Intelligence Committee, working for then-senators Howard Baker and John Tower. He then went to the Justice Department, where he was an aide to then-attorney general Edward Levi.

Afterward, he went to work for Sen. Charles McC. Mathias, who stepped down last year as U.S. senator for Maryland.

The U.S. attorney's office here is the largest in the nation with 214 lawyers and has the unique responsibility of serving as both the local prosecutor for crimes in D.C. Superior Court, as a regular federal prosecutor and acting to represent the U.S. government in civil cases brought in federal court here.