A federal judge in Alexandria yesterday questioned the competence of two U.S. prosecutors who continued to disagree with him over a pretrial ruling in the bribery and tax evasion case of Dr. Murdoch Head, director of Airlie Foundation.

"I'm now sure you'd be competent to try the case," Judge Oren R. Lewis told assistant U.S. Attorneys Joseph Fisher and Theodore Greenburgh. "You can neither read a motion nor understand English Whatever I rule, you're going to tell me you don't understand it."

Lewis, 77, called the case "the most unusual I've had in my 20 years as a judge."

The prosecutors last week appealed to the 4th U.S. Circuit Court of Appeals in Richmond a ruling that Lewis has repeatedly said he did not make. At one point yesterday the judge pounded his desk and sharply cross-examined Fisher and Greenburgh as if he were a prosecutor and they were reluctant witnesses.

The disputed ruling came in a complex criminal case involving allegations that Head bribed Rep. Daniel J. Flood (D-Pa.) in order to win lucrative federal contracts for Airlie Foundation in Warrenton, Va.

The prosecutors contended that when Lewis recently severed the bribery count from other charges set for trial Setp. 10, he also excluded evidence related to a conspiracy charge against Head in his dealings with Flood. The prosecutors appealed the ruling, which Lewis denied making.

"I can't conceive of anyone understanding" the ruling. Lewis said yesterday. "You [both] said you were confused, but I don't believe you . . . I think you're using [the confusion] for a purpose" of delaying the trial.