A Fairfax County Circuit Court jury convicted Susan L. Mullis yesterday of murdering her pharmacist husband on a dirt road in an isolated part of the county where they had gone to discuss marital difficulties. The jury recommended a sentence of 20 years in prison.

The case against Mullis was circumstantial -- there were no witnesses to the slaying, and investigators never found the gun that fired the shots that killed her husband, Michael R. Mullis, 29. Nevertheless, prosecutor Raymond L. Brownelle told the jury that Mullis had the desire, the means and "an abundance of motive" to want her husband dead.

The jury deliberated for nearly seven hours. When its verdict was announced, Mullis, a 28-year-old waitress, lowered her head to the counsel table and rested it there as the jury was sent back to the jury room to deliberate her sentence.

The 20-year sentence recommended is the minimum for first-degree murder; the maximum is life imprisonment. The jury also convicted her of using a firearm in the commission of a felony, for which there is a mandatory two-year sentence.

Judge F. Bruce Bach ordered Mullis jailed pending sentencing Feb. 22. He can reduce, but not increase, the recommended sentence.

Mullis took the stand during the three-day trial to deny any knowledge of how her husband was shot three times in the head on the night of July 23, 1983, on Kincheloe Road in the Clifton area. A former boyfriend testified that she tried several times to enlist him to kill her husband, but she maintained she was only joking.

Mullis testified she left the couple's car to relieve herself in the woods, heard three shots, returned to find her husband shot and ran to get help.

Investigators said the bullets that killed Mullis were fired from a .22-caliber automatic handgun. Assistant Commonwealth's Attorney Brownelle said a weapon such as the one believed used was missing from the couple's home in Woodbridge.

Mullis' attorney, Stephen A. Merril, said his client probably will appeal. Asked if he viewed the minimum sentence as an indication that the jury was not completely convinced of its verdict, he replied: "Absolutely. If they really believed she did it, why did they give her the minimum? It's a hard sentence to understand. I'm not going to second-guess the jury."

Merril said during closing arguments Wednesday that that the evidence was "meager" and insufficient to prove his client guilty beyond a reasonable doubt. "It was zero then at the time of the shooting , it's zero now, and zero plus zero equals zero," he said.

He said no gunpowder residue was found on Mullis' hands or clothes. There was nothing in the case to link Mullis to the crime, Merril said, except her presence in the area.

Merril described Michael Mullis as a drug abuser with severe mental problems.

Brownelle said the couple had been estranged for more than a year, although they were still living in the same house. The killing took place a week before they were to legally separate, at which point Mullis would no longer have been the beneficiary of $225,000 in insurance on her husband's life.

Her conviction means Mullis will not get that money, and Brownelle said he believes at least a part of it will go to the dead man's father.