Joseph N. Martin, a former Northern Virginia life insurance salesman, was convicted of murder yesterday in the slayings of a young Arlington couple in 1977. The Arlington jury that found him guilty then fixed his sentence at life imprisonment, rejecting a prosecutor's request for the death penalty.

The verdict climaxed one of the longest, most controversial and costly prosecutions in Northern Virginia.

Martin sat impassively as the Circuit Court jury pronounced him guilty of murder-for-hire in the death of Alan Foreman, 26, and of second-degree murder in the death of Foreman's fiance, Donna Shoemaker, 25. The couple's bullet-riddled bodies were found in the garage of their North Arlington home in May 1977.

Unless the life term is reduced when Judge Charles S. Russell imposes sentence Jan. 18, Martin, 29, will have to serve a minimum of 20 years before becoming eligible for parole.

The admitted triggerman in the slayings, Richard Lee Earman, has pleaded guilty to murder-conspiracy and can be sentenced to no more than 10 years in prison. Earman, who was acquitted of murder charges in the slayings by a jury two years ago, could not be retried on those charges because of constitutional protection against double jeopardy.

Earman calmly testified at Martin's trial that he shot the couple in the head at point-blank range after the three returned from a Georgetown disco. He said Martin promised him $15,000 for the killings.

The jury deliberated 8 1/2 hours Wednesday night and yesterday before returning its verdict. Martin, who recently had been a salesman in Las Vegas and who had been free under $50,000 bond, was immediately ordered to jail.

Defense attorneys Louis D. Koutoulakos and Gerald F. Teanor Jr. said they plan to appeal.

Special prosecutor Donald S. Caruthers Jr. expressed satisfaction about the verdict. He had argued for the death penalty, saying, "There's a certain amount of depravity of mind about murder-for-hire. If Joe Martin should ever be released, would he do it again? We have no way of knowing."

The defense lawyers called several character witnesses, including Martin's mother, who testified that Martin has recently undergone a religious conversion and is a devout Christian. As several jurors wept, Treanor told them Martin found a "moral anchor through . . . conversion." He added, "having Mr. Martin taken 90 miles south (to Richmond) and killed . . . will not restore the lives of Alan Foreman and Donna Shoemaker."

Defense attorney Koutoulakos cited the maximum 10-year sentence that Earman can receive in urging the jurors to impose a life sentence rather than death.

More than 100 witnesses testified during the trial, which lasted almost seven weeks.

Prosecutors said Martin, an agent for New York Life Insurance Co. at the time of the killings, hired Earman to kill Foreman in order to collect on a $56,000 life insurance policy Martin sold Foreman.

Martin needed the money, according to the prosecution, to pay premiums on $600,000 worth of bogus policies he had fraudulently sold other people in order to boost his own salary and finance a "flashy life style way beyond his income."

The original plan, Earman testified, was to kill only Foreman. But when he told Martin he might have to kill Shoemaker because the couple was rarely apart, he said Martin replied, "beautiful, beautiful," and said a double slaying would divert suspicion from an insurance motive.

Defense attorneys portrayed Martin as an occasional business partner of Foreman and said Martin's insurance fraud scheme was unrelated to the slayings. They also attacked Earman as a cheat and liar. Koutoulakos reminded the jury that Earman testified he had frequently lied to police and prosecutors in order to confuse the authorities and give his own story more credence.

Arlington court officials said prosecution in the murder case, which began with a 1977 trial, will probably cost the taxpayers more than $70,000.

The notorious case has also generated a bitter, two-year feud involving Arlington prosecutor William S. Burroughs Jr., the county police department and Virginia Attorney General J. Marshall Coleman.

In 1977 both Martin and Earman were charged with the murders.

In a surprise move during Earman's trial that year, Burroughs dropped charges against Martin, citing new evidence that he declined to disclose at the time.

Earlier this month, Burroughs, who was replaced by a special prosecutor for Martin's latest trial because he was a prosecution witness, testified he dropped charges because Martin told him he received a threatening note from an unnamed person several days after the murders.

Burroughs testified that the note, which he said he never saw and Martin told him he burned, convinced him that Martin was innocent. Earman was subsequently acquitted by a jury.

In 1978, Arlington police and others complained to Virginia Gov. John N. Dalton about Burroughs' handling of the case and what they said was his reluctance to prosecute Martin. Coleman subsequently ordered a three-month state police probe of the prosecutor's conduct. That probe cleared Burroughs of criminal misconduct.

Since then, Burroughs, a Democrat, and Coleman, a Republican, have essentially investigated each other. Last year Burroughs ordered two Arlington sheriff's deputies to investigate why Coleman ordered the state police probe.

Burroughs, who was defeated last month in a bid for reelection, has been rebuffed three times this year in his bid to get a special grand jury to investigate Coleman. Three Arlington Circuit Court judges have declined to impanel a special grand jury for the probe after a majority of jury members said they would not serve on the panel.

Burroughs has said he plans to make a fourth such request when a regular grand jury convenes next week. Six weeks ago when an earlier grand jury was considering Burroughs' request, Coleman filed unusual papers in Arlington Circuit Court detailing for the first time why he ordered Burroughs investigated.

In those papers, Coleman said he had received "corroborated information" that the prosecutor may have committed bribery by refusing to prosecute Martin. Other allegations, Coleman said, were that Burroughs intervened in a lie-detector test "in a manner which may have benefited Martin" and that Burroughs' "apparently close relationship with Martin . . . may have constituted malfeasance in office."

Burroughs called a press conference to deny those allegations, citing the advice of a former colleague, who he said advised him, "Don't get into a p---ing contest with a skunk."