Five times between January 1977 and February 1978, a Russian man approached cars with American diplomatic license plates in Moscow, begging to speak to an American.

By chance, the first car he encountered at a gas station belonged to the Moscow station chief of the Central Intelligence Agency. Fearing that the Russian was a KGB agent sent to entrap Americans, his request for a meeting was ignored.

But the persistent Russian kept coming back, once pounding on the station chief’s car. Each time, he revealed more about himself, indicating that he had information about Soviet weapon systems. Each time, he was turned away.

Finally, he made one last desperate effort to reach out to the Americans. If he was spurned this time, he wrote in an 11-page letter, he would give up. The CIA assigned a Russian-speaking officer named John Guilsher to make contact with the man, a senior engineer named Adolf Tolkachev.

After several telephone conversations, Tolkachev and Guilsher met in person on New Year’s Day 1979. Tolkachev passed along 91 pages of notes about his work with Soviet radar systems and aircraft. He also impressed Guilsher because he seemed to be one of the few sober Russians walking the streets of Moscow on the festive holiday.

Thus began one of the most remarkable episodes of espionage in the history of the Cold War.

“For almost a decade,” intelligence expert David Wise has written, “Tolkachev proved to be the CIA’s most valuable asset inside the Soviet Union, his existence a closely guarded secret.”

Guilsher’s role in this true-life spy tale was just as remarkable and was never fully revealed until last year, when retired CIA officer Barry Royden published an official account of the clandestine mission.

“John Guilsher’s story was extraordinary,” CIA spokesman Mark Mansfield said. “He was a legendary case officer who put himself at great personal risk to deal with one of the most valuable and productive agents in CIA history.”

John Ivan Guilsher was born July 10, 1930, in New York and spoke Russian at home with his parents, who were descended from Russian nobility and had lost their homes, their careers, and their entire way of life in the Bolshevik Revolution of 1917.

Guilsher graduated from the University of Connecticut and served as an Army intelligence officer during the Korean War. He joined the CIA in 1955 and had assignments all over the world before being posted to Moscow in 1977.

His efforts to gain information from Tolkachev opened a window on the CIA’s shadowy methods of spycraft. He and other CIA officers concealed coded information, cash and matchbox-size cameras in a dirty mitten, seemingly discarded on a street. A light in a window or a car parked in a certain direction could bring, or cancel, a face-to-face meeting.

Under constant surveillance by the KGB, Guilsher became a master of disguise. He drove to the U.S. Embassy for a dinner engagement, then left through a back door and climbed into another car.

“While still in the vehicle,” Mr. Royden wrote in the CIA report, Guilsher “changed out of his western clothes and made himself look as much as possible like a typical, working-class Russian by putting on a Russian hat and working-class clothes, taking a heavy dose of garlic, and splashing some vodka on himself.”

Guilsher then walked or rode buses and subways to his secret meetings. He later went back to the embassy and emerged from the front door in a diplomat’s suit and tie.

They met more than a dozen times, as Tolkachev turned over hundreds of rolls of film and written specifications of Soviet aircraft, missile systems, and electronics. In return, he asked for medicine, books, Western razor blades, and rock ‘n’ roll tapes for his teen-age son. He requested a poison pill, in case he should be caught by the KGB, but the CIA said no.

Tolkachev also asked Guilsher for antique jewelry and money, suggesting that “several million dollars is not too fantastic a price.” In the end, the CIA agreed to pay him a stipend “equivalent to the salary of the U.S. President,” with most of it held in escrow for the time when Tolkachev would defect. It turned out to be a bargain.

“From January 1979 until June 1980, Tolkachev had provided an extremely high volume of incredibly valuable intelligence to the US military,” Mr. Royden wrote in the CIA report. “This information could have meant the difference between victory and defeat, should a military confrontation with the USSR have occurred.”

In mid-1980, Guilsher left Moscow for another assignment. His espionage role was never discovered by the KGB. “He had made Tolkachev feel confident,” the CIA account concluded, “and the two had become comfortable with each other in the dangerous endeavor in which they were involved.”

Tolkachev continued to meet other American operatives periodically until early 1985, when a rogue CIA officer named Edward Lee Howard, who was fired for theft and abusing drugs, identified Tolkachev and other secret agents to Soviet authorities. Tolkachev was arrested in June 1985 and executed in September 1986.

“My husband had so much admiration for Tolkachev,” Catherine Guilsher said. “He took it very heavily.”

Howard disappeared into the Soviet Union and died mysteriously in 2002 at age 50.

Guilsher worked at the CIA until 1990, then continued as a consultant on Russian matters for the agency until last year. He was 77 when he died April 4 of pancreatic cancer at his home in Arlington, Va.

In addition to his wife of 50 years, survivors include three children, Michael Guilsher of Birmingham, Ala., Anne Guilsher of Arlington, and Alexandra Guilsher of New Marlborough, Mass.; a sister; and two grandchildren.