Bloch stayed with Ely for two nights. On one of those evenings, he had supper out. ''Don't worry about me,'' Bloch told his colleague, ''I'm having dinner in town with an old friend.'' Ely, a polite Princeton man, insisted on dropping Bloch off at the Hilton. He remained in his car and did not see whom Bloch met.

Bloch walked through the lobby, up a long, curved, carpeted stairway and pushed open the glass doors of La Maison du Boeuf, a dining room of soft banquettes and bright chandeliers. There a familiar, bearded companion joined him for dinner. Once again, Bloch told me, he handed materials to Pierre Bart. Stamps? He nodded, yes. The Brussels meeting created a predicament for the F.B.I. and its intelligence division in Washington. From the tap on Bloch's phone, it knew he was going to Brussels. It could not afford to let another meeting with Gikman, the Soviet agent, take place unobserved. Only two weeks earlier, French agents had provided surveillance for the Paris meeting. Logically, the Belgian Surete de L'Etat would be asked by the F.B.I. to do the same in Brussels, but that request was never made.

''We've felt the Belgian service has been penetrated by the Soviets for a long time,'' said one United States intelligence officer close to the Bloch case. There may be other explanations as well. Albert Raes, the prickly chief of the Surete, does not enjoy good relations with the other Western security services.

The F.B.I. asked the C.I.A. to provide surveillance of the meeting between Bloch and Gikman, and it did, albeit reluctantly. It was a very risky business. ''The fact that the agency covered was very sensitive,'' an intelligence source said. ''The C.I.A. was running a covert operation without telling the host country. It was very dicey. The whole station could have gotten kicked out.'' But the C.I.A.'s surveillance in Brussels may have been discreet to a fault; there was confusion about whether Bloch brought a bag to the meeting, and whether he left it with Gikman. Weeks later, when investigators questioned Ely, they went over and over the same point: had Bloch thrown a bag in the car when he left Ely's house? Try as he would to recall, Ely could not be sure. And he did not know if Bloch carried anything when he returned to the house after dinner. The Elys had retired early and Bloch, who had his own key, let himself in. Ely realized from what he was asked that the meeting had been observed, and he must have wondered at the questions. Didn't they know? But the C.I.A., reluctant to reveal its hand, may have hoped that the agency's surveillance would remain hidden if Ely could provide the needed evidence firsthand. He could not.

President Bush called on Jacques Delors on May 30, and his plane left Brussels at 3:10 P.M. Immediately afterward, Bloch left by train for Paris. He attended another economic conference, spent the weekend in Paris, and flew home.

JUST AFTER 6 A.M. ON Thursday, June 22, Reino Gikman called Bloch at his home in Washington. To the listening F.B.I. wiretappers, the call was clearly a warning. ''He did not use the word virus, it's garbled in the press accounts,'' Bloch said. ''But he said he wasn't feeling well and hoped I didn't catch the same thing. I said to him, 'I hope it's not serious.' 'No, it's not,' he said. I wished him well and hung up.''

By even the most charitable interpretation it seemed unlikely that Gikman, after three weeks, was really worried that his friend would catch the flu. James H. Geer, then the F.B.I. counterintelligence chief, realized with dismay that the Soviets had somehow learned that Bloch and Gikman were under surveillance. How is not clear. The most ominous explanation - the tipoff came from a mole inside United States intelligence - is not necessarily the only one. Soviet agents watching Bloch's apartment in Kalorama, for example, might have detected the F.B.I. surveillance.