A federal judge dismissed a tax-evasion charge yesterday against Dr. Murdock Head, the Airlie Foundation founder who was convicted earlier this month of participating in a criminal conspiracy.

A District Court jury in Alexandria had reported itself "hopelessly deadlocked" on the tax-evasion count. The jury convicted Head on Oct. 12 of a single conspiracy charge, but acquitted him of two other tax-evasion charges.

At a hearing yesterday morning, Judge Oren R. Lewis dismissed the remaining tax-evasion charge against Head -- the count on which the jury was deadlocked. Lewis said there was insufficient evidence to warrent a new trial on the charge.

At the same time, Lewis urged prosecutors to try Head later on a separate bribery count. The bribery charge alleges that Head gave a $1,000 payoff to Rep. Daniel J. Flood (D-Pa.) in 1974. Lewis recommended that Head be tried on this charge at the same time that Flood's second trial takes place.

Flood's first bribery trial ended in a hung jury last February. A retrial has been delayed because of Flood's poor health. Flood's second trial is now scheduled to begin in Washington in December.

The conspiracy charge on which the 55-year-old Head was convicted alleged that Head schemed to bribe two members of Congress and to violate tax laws. The Washington Post has reported, however, that the jury did not believe Head was guilty of conspiring to bribe the two, Flood and former representative Otto E. Passman (D-La.).

Instead, The Washington Post reported, the jury interpreted the conspiracy count more narrowly. The Post reported that the jurors concluded Head was guilty of taking part in a conspiracy to commit tax infractions, including arranging an improper $11,000 loan to a former Internal Revenue Service agent.

Federal prosecutors said yesterday they had not yet determined whether they would appeal Lewis' dismissal of the tax-evasion count against Head.

U.S. Aorney Justin W. Williams said he would also consult the Justice Department and the U.S. attorney's office in the District of Columbia before deciding what action to take on the bribery count.

Head has temporarily stepped aside as executive director of the 19-year-old Airlie Foundation, a tax-exempt educational institution based near Warrenton, Va. Head has also temporarily relinquished his day-to-day duties as chairman of the George Washington University department of medical and public affairs.

Head's lawyers have announced plans to appeal his conviction on the conspiracy count. Head is scheduled to be sentenced Wednesday on the charge by Judge Lewis. Lewis previously dismissed eight other bribery and tax-falsification counts on which Head had been indicted.