A former boyfriend of Susan L. Mullis testified yesterday, as Mullis went on trial on a charge of murdering her husband, that she tried several times to get him to arrange her husband's slaying or kill him himself.

"A week after we met . . . she asked me if I knew anybody who would kill him," Michael S. Young, a 31-year-old auto painter from Alexandria told a Fairfax Circuit Court jury. At another point he said, "She asked me to do it, that's all I know."

Young said he "didn't take Mullis seriously" when she spoke of having her husband killed.

Michael R. Mullis, a 29-year-old Woodbridge pharmacist, was shot to death July 23, 1983, in an isolated part of Fairfax County near the town of Clifton where he had driven with his wife.

Susan Mullis, 28, charged with first degree murder and use of a firearm in the commission of a felony, has maintained her innocence.

She told police the night of the shooting that she had gone into the woods to relieve herself, heard three shots, returned and found her husband shot and ran to summon help.

According to yesterday's court proceedings, the killing took place a week before the Mullises were to begin a legal separation as a step leading to divorce and Mrs. Mullis was the beneficiary of $200,000 in insurance on his life at the time.

Police have found no murder weapon and have no witnesses to the killing. Assistant Fairfax County prosecutor Raymond L. Brownelle told the jury yesterday, "This is a classic circumstantial case . . . but given all the evidence you'll hear, I think the sum of the circumstances will prove it beyond a reasonable doubt."

The defendant's attorney, Steven A. Merril, told the jury, "Susan Mullis did not kill her husband and she doesn't know who did," and she has cooperated completely with police.

He characterized Mullis as a man with "severe mental problems who believed he had worms coming out of his bowels." Merril said Mr. Mullis abused drugs and was "very paranoid."

"They have no evidence to prove Susan Mullis is guilty beyond a reasonable doubt. Their investigation raises more questions than it answers," Merril said.