Arlington trial attorney Claude M. Hilton was named a special prosecutor yesterday to probe "alleged prosecutorial misconduct, including criminal activity" by Alexandria Commonwealth's Attorney William L. Cowhig.

Hilton, 38, who served as Arlington's chief prosecutor in 1973 and 1974, met immediately with Alexandria's Acting Commonwealth's Atorney John E. Kloch at Kloch's City Hall office to discuss the focus of the investigation.

Hilton was appointed by Circuit Court Judge Wiley R. Wright amid allegations that Cowhig, while Alexandria's chief prosecutor in 1975, solicited a sexual favor from a defendant's wife. Hilton also is expected to look into charges that results of a police investigation of the incident were covered up.

In a separate development, Cowhig defense lawyer Blair D. Howard met yesterday with Judge Wright and Alexandria's special bingo prosecutor, Edward J. White. Howard said later that under an agreement reached by the three, Cowhig will resign as chief prosecutor effective Feb. 23. A letter of resignation will be delivered today to Wright, Howard said.

In return, White has agreed to drop the last of three bingo-related felony charges against Cowhig. Cowhig was scheduled for trial on the charge on March 5. Details of the arrangement were made public last week.

Cowhig said last weekend he plans to enter private law practice and will not seek reelection.

Cowhig was acquitted of the other two charges in separate jury trials in Alexandria Circuit Court in December and January. He consistently has denied any wrongdoing in the bingo cases, and has echoed that position in his statements about the sex allegations that surfaced last week.

Hilton, who will work out of his private law office on Wilson Boulevard in Arlington, said he will review documents, including taped conversations with the defendant's wife, before deciding what charges, if any, can be brought.

"There are witnesses to interview, people to talk to," Hilton said.

Hilton's new assignment is not his first as a special prosecutor in Alexandria. In 1977, he was appointed to investigate the city's sheriff, Raymond E. Fogle, who was charged with encouraging his employes to pad their expense accounts.

The investigation was terminated when Fogle left office and money was repaid to the city as a result of the probe.

Hilton, who served as an assistant prosecutor in Arlington in 1976, was named chief prosecutor in 1973 after William J. Hassan retired because of ill health. He was defeated in the following year by William S. Burroughs Jr., the current incumbent.

Hilton was praised yesterday by one Northern Virginia attorney, who described him as a vigorous prosecutor. Hilton is a former law partner of Louis Koutoulakos, one of Cowhig's lawyers in his first bingo trial. Hilton said yesterday he has known Cowhig "for years."

The latest allegations against Cowhig coincided with mounting pressure on the 53-year-old prosecutor to resign his office despite his earlier acquittals. A letter to Cowhig signed by Kloch and four of six assistant prosecutors threatened a mass resignation if Cowhig resumed his duties. Cowhig had stepped aside from office temporarily following his indictment last Aug. 3.

According to Judge Wright's order, Hilton's duties will be limited to offenses "in any way connected" with the cases of Daniel Wayne Clenault and his wife, Sherry Rea Chenault.

Chenault reportedly told Alexandria authorities that in 1975, while her husband awaited trial on drug charges. Cowhig offered leniency in exchange for a sexual favor from her.

According to Chenault, who has been granted immunity from prosecution for her testimony, she met with Cowhig in his third-floor City Hall office and performed an act of oral sodomy.

Cowhig has said that the meeting occurred, but contended that Chenault tried to seduce him.

Results of a lie detector test administered to Cowhig by Alexandria police at the time allegedly were misrepresented to local prosecutors, sources said last week.