In any case, he had already made up his mind. Soon he was volunteering information about his associates in the K.G.B.'s local office and disclosing how Soviet officers were listening in on F.B.I. transmissions.  His American handlers were confident that he was not feeding them disinformation. "He couldn't lie," said one of them, Bill Smits, since retired, "because he didn't know how much we knew."  Mr. Yuzhin says he was not motivated by money, and Mr. Smits says the F.B.I. did not pay Mr. Yuzhin at the time. As for any financial arrangements later, the bureau declines as a matter of policy to discuss them, and Mr. Yuzhin is also silent. A Crucial Mistake  After 10 months, Mr. Yuzhin returned home. But two years later, in July 1978, as the bureau had confidently predicted, he was back in San Francisco for the K.G.B., this time acting as a reporter for the Soviet news agency Tass and serving once again as a leading source of information for the F.B.I.  He had one close, if comic, brush with disaster. The F.B.I. had given him a new spy camera concealed in a cigarette lighter. He put it in his back pocket and, within hours, lost it -- in the auditorium of the Soviet Consulate, as it turned out.  It was found by a handyman who, trying to light a flame, took four pictures of his own face and soon discovered the gadget's purpose. The K.G.B. undertook a frantic mole hunt, matched by the F.B.I.'s equally frantic, and unsuccessful, effort to recover the device. But Mr. Yuzhin escaped immediate suspicion.  On another occasion, F.B.I. agents say, Mr. Yuzhin snapped photos with a miniature camera inside the local K.G.B. office, unaware that he was also photographing a reflection of himself in a mirror. That accident meant that his role as an American agent might now be known not only by his handlers in the highly compartmentalized world of counterintelligence but also by anyone who happened to gain access to the photos. It was an accident that may have led to Mr. Yuzhin's undoing: people familiar with Mr. Ames's debriefings by the F.B.I. say that at some point he apparently became aware of it.  Meanwhile, Mr. Yuzhin continued supplying information, managing, Mr. Smits said, to "Xerox the annual report of the K.G.B. political branch" and smuggling out crucial cable traffic. He also tipped the F.B.I. to the existence of a Soviet spy somewhere "up north," a lead that helped Norway break the espionage case of a Foreign Ministry official, Arne Treholt, arrested in 1984 and sentenced to 20 years in prison. "It was one case we were giving up on," Mr. Smits said. The Unraveling  In 1982, apparently as a matter of routine, Mr. Yuzhin was called home for debriefing and what turned out to be a series of lackluster assignments in Moscow. Before he left, the C.I.A. tried to arrange for contact with him there. But he refused, fearing it too risky.  On Dec. 23, 1986, he was summoned by his chief and ordered to the airport for an errand. There, he said, he was shoved into a room, handcuffed and held incommunicado on charges of high treason. His wife, Nadya, had had no inkling of his double life and was as stunned as anyone, she said in an interview here, speaking in halting English.  He had been under suspicion and surveillance for some time, he learned later. A video camera had even been hidden in his Moscow apartment, recording his every conversation and intimate moment with his family.  Yet his interrogators' questioning, Mr. Yuzhin says, showed that some of their information was spotty. He could, in fact, refute accusations that he had been in contact with the C.I.A. after returning to Moscow. As for the rest, he said, "I painted a terrible picture of the F.B.I.," contending that the bureau had exploited him ruthlessly, a portrayal that he said the Soviet authorities were predisposed to accept.  He was convicted and sentenced to 15 years' "strict regime." For a time during his imprisonment, he occupied the cell in which a notable dissident, Anatoly Marchenko, had just died of mistreatment. The F.B.I. lost track of him and gave him up for dead.  In February 1992, the amnesty granted by Mr. Yeltsin freed Mr. Yuzhin from a labor camp, Perm 35 in the Urals. A speaking invitation from the World Affairs Council of Northern California afforded the Yuzhins and their teen-age daughter, Olga, the opportunity to leave Russia two years ago, while a grown son remained behind.  Since then, Mr. Yuzhin has eked out a living in the Bay Area as a writer and archivist cataloguing documents of the Soviet era and researching the cases of prisoners of war who disappeared in the Soviet Union, including the anti-Nazi war hero Raoul Wallenberg.  "I never regretted what I did," he said. Indeed, he said, his imprisonment confirmed his belief in the course he had chosen, showing him as it did a side of the Soviet state that he had never witnessed in his privileged position as a K.G.B. officer. "The more I thought, the more I realized I did the right thing," he said, "because I got another taste of the system."  Photos: After five years' imprisonment in the old Soviet Union, Boris Yuzhin is back in the San Francisco area, where he once alerted the F.B.I. to Soviet spies' drop sites like this one in Christopher Park. (Jim Wilson/The New York Times); Boris Yuzhin was a K.G.B. lieutenant colonel when this photograph of him was taken in 1982.
"I still remember, I remember this town," Boris Yuzhin murmured, looking for landmarks as he piloted his Dodge van up the steep curves of the Diamond Heights section here. Recognizing a small shopping center, he veered off, parked the van outside a pizzeria and circled on foot through Christopher Park to a secluded pathway among cedars and pines. There, on his knees, he pried at the planks of a wooden staircase, searching for holes.   In the late 1970's and early 80's, Mr. Yuzhin and other officers of the K.G.B., the Soviet intelligence agency, scouted this "drop" and others like it in the Bay Area as places to hide microfilm for pickup by other Soviet spies. Then, as a double agent, he disclosed those locations, and much other information, to the F.B.I.