A sobbing 15-year-old girl from Costa Rica testified yesterday in Montgomery Circuit Court that her adoptive American father began a 2 1/2-year pattern of sexually molesting her days after he brought her and her little brother to this country in 1981.

The alleged pattern of abuse she described in detail yesterday was nearly identical to the testimony in August of her older sister, 25, who also accused her father of forcing himself on her sexually two to three times a week over a six-year period that began when she was in fifth grade.

Under questioning by prosecutor Barry Hamilton yesterday, the 15-year-old daughter testified that her father hit her when she refused his advances. "Sometimes I'd refuse, but he'd tell me he'd spank me if I didn't do that . . . . I asked not to, to please stop doing it. I don't want to do it anymore," she said, weeping.

The man, whose name is not being used in this story because it is shared by the adopted girl and by some of her sisters, was convicted of child abuse in the August case and sentenced to 10 years in prison. In addition to the allegations brought by the adopted girl, the man also faces separate charges of child abuse or rape brought by his two other natural daughters.

Under a ruling by Judge Calvin R. Sanders, the seven-man, five-woman jury has not been made aware of the other charges facing the 51-year-old man, nor of his conviction earlier this year.

The father, a successful computer operator who retired nearly nine years ago, apparently was able to hide his allegedly incestuous relationships from the mother, who testified yesterday that she was unaware of any problem with the adopted daughter.

Recounting one incident late at night while her mother was asleep, the girl testified that her father, red-eyed and reeking of alcohol, tried to force her into a sexual act.

"He was acting crazy. He woke me up," and ordered her to perform a sexual act in the downstairs guest room, she said, her testimony punctuated by tears. "I refused and he hit me in the stomach. I couldn't breathe . . . . I tried to scream, but he put his hand over my mouth."

When her father let her go, she said, she ran upstairs to a bathroom near her parents' room. She said her mother woke up and asked her if she was well. " 'Of course I'm okay,' I said," she told the court.

Asked by Hamilton why she never complained to her adoptive mother about the man's attacks, the girl replied, "I thought she would believe my father because she's been with him for 27 years." But she also said she'd always thought that her father would "get caught."

The girl testified that she also had been sexually molested in Costa Rica -- by her mother's landlord who also molested her little brother and by her mother's boyfriend. It was that abuse that prompted authorities to take the children and place them up for adoption.

Defense attorney Cleopatra Campbell sought to get the girl to admit that she was confusing the sexual abuse in her homeland with her adoptive father's loving care. But the girl, weeping and stumbling often in her command of English, stuck to her story. Once when Campbell accused her of not always telling the truth, the girl shot back, "Why would I lie . . . about this?"