A 29-year-old woman who has accused her father, a University of Maryland professor, of sexually abusing her over nearly two decades took the witness stand in a Prince George's County courtroom yesterday and described several of the alleged incidents in detail.

Angela Mattson, who is now married and lives in Kansas, was the opening witness in the Circuit Court trial of her father, Frank J. Munno. He was director of the university's nuclear engineering program at College Park before he was indicted in June.

Mattson, who also is a nuclear engineer, testified that Munno tricked her into performing oral sex when she was 8, beginning an 18-year incestuous relationship that continued until she was 26.

"I didn't like what was going on one bit, but it was my father's house and his rules," Mattson said, when a prosecutor asked why she had not forcefully resisted her father's alleged advances. Mattson also testified that she worried about "losing his love." And she added that she did not understand, even after reaching adulthood, that incest was illegal.

"I never believed my father was doing something wrong," she said under cross-examination. "It never entered my mind that my father would do something wrong to me."

Although she described several of their alleged encounters in detail, she was unable in many instances to recall what day of the week, what month or season, or even what year they occurred.

Munno, 55, a former Navy officer and a member of the College Park faculty for 32 years, has pleaded not guilty to one count each of child abuse, incest and unnatural and perverted sex acts. The indictment covers a range of offenses allegedly committed between 1969 and 1987.

The university said Munno, a tenured $89,000-a-year professor, had been placed in a non-teaching job until the charges against him are resolved.

Mattson, who lived in Bowie and worked for the Nuclear Regulatory Commission at the time of her father's indictment, gained wide media attention by telling her story publicly and allowing her name to be used. Usually news media do not identify victims of sex crimes by name. Mattson said she hoped to inspire incest victims to come forward and file complaints.

Yesterday, however, Munno's attorney, Karen Silver, suggested to jurors that Mattson's allegations against her father resulted from a financial dispute between them. In her opening statement, Silver urged jurors to "keep an open mind" as they listened to Mattson's testimony, and to ask themselves why Mattson would have kept such sexual abuse a secret for so many years, and why she would not have strongly resisted her father.

Silver also noted that the prosecution had found no physical evidence or corroborating witnesses to support Mattson's allegations.

Prosecutor Beverly Woodard, in her opening statement, conceded this lack of evidence and said Mattson's credibility would be "key to this case."

"Ask yourselves, why would she make this up?" Woodard said. "Why would she tell 12 strangers that her father had sex with her?"

Mattson, who has a twin brother and an older brother, testified that she was living with her parents in their College Park home when the alleged sexual relationship with her father began in 1969. She testified that she had oral sex with her father routinely, mostly when her mother and brothers were not home. "As I got closer to 13, he started to talk about intercourse," Mattson said.

She said they had intercourse for the first time on her parents' bed while her mother was away, and developed a routine of having sex most nights on a living room couch while the rest of the family slept upstairs.

The alleged relationship continued until she left home at age 26, after finishing graduate school at College Park. She said she told no one about the alleged abuse except her twin brother, in whom she confided in the early 1980s, and her husband, whom she married last year.

In an interview in June, Mattson said her husband persuaded her to seek psychological counseling. Early this year, after months of therapy, Mattson said, she decided to file criminal charges against her father.

Defense attorney Silver continually suggested that Mattson, who is trained in karate, had the physical ability to resist her father. And by showing that Mattson had a college degree and thousands of dollars by the early 1980s, Silver sought to show that she had the financial means to leave home.

But Mattson said, "He was my father and I did as I was told . . . . I had many years of training as being an obedient child."