WASHINGTON -- President Clinton yesterday fired William S. Sessions as director of the Federal Bureau of Investigation. The president is expected to replace him today with Louis J. Freeh, an ex-FBI agent who is now a federal district judge in New York.

Mr. Clinton telephoned Mr. Sessions yesterday afternoon to tell him he was being dismissed immediately for what Attorney General Janet Reno termed "a serious deficiency in judgment" that had destroyed his ability to lead the agency.

Judge Freeh, 43, is a lifelong crime fighter best known for convicting 18 members of the billion-dollar "Pizza Connection" drug conspiracy. Mr. Clinton spent two hours alone with Mr. Freeh Friday evening, ending with a personal tour of the White House. The president has interviewed no other candidate for the job, said Dee Dee Myers, the White House press secretary.

If he is confirmed by the Senate, Judge Freeh could be a soothing balm to the FBI, which has been scarred by bitter feuds between the mild-mannered Mr. Sessions, who was often described as ineffectual, and a resentful coterie of veteran agents and senior officials, some dating to the days of the agency's first director, J. Edgar Hoover.

Although both Mr. Sessions and Judge Freeh have served as federal judges, their styles and backgrounds are quite different.

Mr. Sessions, 63, spent only four years, 1971-74, prosecuting criminals as U.S. attorney in Western Texas. The rest of his career consisted of four years in the Air Force during the Korean War, 11 years as a lawyer in Waco, Texas, three years handling civil cases in the Justice Department in Washington and 14 years as a U.S. district judge in Texas.

Judge Freeh, on the other hand, has devoted most of his adult life to chasing crooks. Right out of law school he put in five years as an FBI agent concentrating on labor racketeering, followed by two years in a supervisory post at FBI headquarters in Washington and 10 years as a federal prosecutor in New York specializing in organized crime.

'The best person'

Judge Freeh's former boss as U.S. attorney in New York, Rudolph Giuliani, a Republican candidate for mayor, told Newsday: "This is a person who I can honestly tell you -- and I know the FBI very well -- is the best person to run the FBI. . . . He knows the bureau from the ground up."

In 1991, President George Bush nominated him to be a federal judge in the Southern District of New York.

Mr. Session's tenure was put in doubt in January when the Justice Department's Office of Professional Responsibility (OPR) issued a stinging report rebuking the director for violating the bureau's ethical guide-lines.

Mr. Sessions, whom President Ronald Reagan appointed in 1987, indignantly denied the charges and resisted increasingly obvious hints from the White House that he should resign.

'Old boy' network

Along with his wife, Alice, he blamed an "old-boy" network of veteran FBI agents and officials for resenting his efforts to bring more women and minorities into the bureau.

The FBI had historically been an all-white, all-male organization. It took in its first female agents in 1972, two months after Mr. Hoover's death, but still today, only one in 10 agents is female. Blacks and Hispanics continue to complain of discrimination, and Mr. Clinton took pains yesterday to praise Mr. Sessions' campaign for diversity in the FBI.

"I think that will be remembered as the best thing about his tenure," the president said.

Mr. Sessions' fall was essentially self-inflicted; he gave his enemies ammunition when he disregarded bureau rules in such things as using FBI planes to visit relatives and spending government funds for a $10,000 fence outside his home.

Attorney General Reno said yesterday that, after reviewing the matter for four months, she had concluded that Mr. Sessions "exhibited a serious deficiency in judgment involving matters contained in the [Justice Department] report . . ."

Mr. Clinton, standing at Ms. Reno's side in the White House press room, said, "I fully agree."

Mr. Sessions said yesterday that he had refused to resign "because I believe in the principle of an independent FBI" and suggested that resignation under pressure would amount to allowing the bureau to be "politicized."

None of the six men who have headed the FBI since Hoover died has lasted the 10 years allowed by a law passed to prevent the director from acquiring the abusive power Mr. Hoover wielded in his 48-year tenure.