WASHINGTON, Feb. 7 — The television journalist Tim Russert testified today that, contrary to what I. Lewis Libby Jr. has maintained, the two of them never discussed a C.I.A. agent whose unmasking touched off a scandal.

“No, that would be impossible,” Mr. Russert said when the prosecutor, Patrick J. Fitzgerald, asked him if he had told Mr. Libby about the agent. It was impossible, Mr. Russert said, because he did not know about the agent at the time of a crucial conversation with Mr. Libby.

Did Mr. Russert tell Mr. Libby that “all the reporters” knew about the agent, asked Mr. Fitzgerald, as Mr. Libby has maintained Mr. Russert told him.

“No,” Mr. Russert said, repeating the same one-word answer to several similar questions from Mr. Fitzgerald, who is trying to prove that Mr. Libby lied to investigators and grand jurors during an inquiry into who leaked the name of the covert agent.