A woman testified yesterday in the trial of a District police officer charged with sodomy and bribery that she performed a sex act on the officer in a parking garage minutes after he asked her if she had $100.

Lois Frontuto, 33, who told a D.C. Superior Court jury that she has worked as a prostitute here since 1980, testified she thought officer Albert Sell's remark to her moments after he saw her with two customers meant that she was going to be arrested on prostitution charges and that $100 was the amount of bail she would have to post.

Frontuto said of the September 1980 incident, however, that she knew something "unusual" was happening when the officer did not handcuff her and told her to sit in the front seat of his car. She said she then drove with him to a parking garage and performed a sex act.

Sell is also charged with obstructing justice in connection with an incident several months later in which it is alleged he tried to influence the woman's testimony before a grand jury. According to a tape recording of the incident that was heard in court yesterday, a man identified as Sell told Frontuto that if he was brought to trial and a court finds "something on him" his friends on the police force would lock her up "right and left."

Sell's lawyer said in his opening statement that he would prove that Frontuto was a vindictive woman who would do anything to "ply her illegal trade," including making false allegations against Sell.

"Her ulterior motive was to get the cops off her back," said attorney Robert E. Greenberg.

Sell, of Gaithersburg, has been suspended without pay since his indictment. He faces a maximum sentence of 18 years in prison if convicted.

Frontuto said yesterday she had just finished with two customers near 16th and R streets NW when Sell approached and asked for identifications. She said Sell told the men and another woman to leave and then escorted her to his police car.

Frontuto said she knew she was not going to the police station when they passed a turnoff to the police station.

"I asked him where we were going and he said we were going to find a place to park," Frontuto testified. They drove to a parking garage, where she said she performed a sex act on Sell after asking him what he wanted.

Sell, she said, later dropped her off at a downtown street corner. Frontuto said she "felt rotten" and told another police officer what happened. That officer radioed for help and Sell appeared. Frontuto said she told the other officer that Sell was the officer involved.

Frontuto said she did not file a complaint that night because she wanted to get home and that she felt her statement would not be taken seriously. She said she changed her mind later and agreed to file a formal complaint.