Frank J. Munno, a University of Maryland professor accused of sexually abusing his daughter for nearly two decades, once told his elder son during a "philosophical discussion" that he considered incest to be morally acceptable, according to trial testimony yesterday.

The son, John Munno, who is in his early thirties, appeared as a prosecution witness on the second day of his father's Circuit Court trial in Prince George's County. He recalled that the discussion about incest took place in his high school or early college years.

He said his father told him that incest had come to be viewed as immoral long ago because children born of such relationships sometimes suffered genetic defects. "But now, with birth control, it wasn't a moral issue," John Munno said, allegedly paraphrasing his father. "It was okay to have incest."

Frank Munno, 54, was director of the university's nuclear engineering program at College Park before being indicted in June. He is now in a nonteaching job until the charges against him are resolved.

His daughter, Angela Mattson, 29, who is married and lives in Kansas, has testified that her father engaged her in a variety of sex acts from the time she was 8 until shortly before she left home in 1987 at age 26.

Munno's attorney, Karen Silver, called a series of witnesses yesterday, including Mattson's twin brother, C. James Munno; some of Frank Munno's neighbors in College Park; a former babysitter; and several relatives and family friends. None could recall any unusual behavior involving Munno and his daughter.

But under cross-examination by prosecutor Beverly Woodard, all acknowledged that there probably were many times when Munno and his daughter were alone together.

Silver then rested the defense's case without calling Munno or his wife to testify. Closing arguments are set for this morning.

Mattson, also a nuclear engineer, gained wide media attention after her father's indictment by telling her story publicly and allowing her name to be used. Although the news media usually withhold the names of alleged sex crime victims, Mattson said she hoped that by sacrificing anonymity, she would inspire incest victims to file criminal complaints.

With Frank Munno choosing not to testify, yesterday's court session boiled down to a case of one brother's word against another's: After John Munno had testified for the prosecution, his younger brother took the witness stand in their father's defense.

C. James Munno, known as "Jimmy," said that he and his twin sister were "very, very close" until the last few years and that she "shared her deepest, darkest secrets with me." He added that "the nature of the relationship was such that she confided to me intimate sexual details" regarding her relationships with various boyfriends.

Yet she never so much as hinted at an incestuous relationship with their father, he said. With that, he contradicted his sister, who testified Tuesday that she told him about it in 1983.

John Munno, when asked about his sister and father, said he once "observed them French-kissing, or tongue-kissing each other, which I thought was unusual." Under cross-examination, he said that the kiss "lasted two seconds" and that he never witnessed them in a sex act.

Mattson was unable Tuesday to pinpoint the exact dates of many of the alleged sexual acts with her father, but did describe encounters she said occurred on her 16th birthday, on New Year's Eve 1985, and in the early weeks of July 1987. Jimmy Munno, however, disputed her accounts, saying that their family was together much of the time and that he saw nothing unusual.

He said he believes his father is innocent of the charges, one count each of child abuse, incest, unlawful carnal knowledge, and unnatural and perverted sex practices. The most serious offense, child abuse, is punishable by up to 15 years in prison.

John Munno seemed less sure of his father. "I still love him," he said from the witness stand. "I think he was a good father to me and my brother. But the last few years, I've been feeling a conflict between emotions and memories. I have a lot of great memories, and a lot of sad emotions."