Just four months after being convicted of sexually abusing his second oldest daughter, a 51-year-old former computer operator sat impassively yesterday as a Montgomery County Circuit Court jury convicted him of molesting his youngest daughter.

Deputy sheriffs immediately hustled him into a holding cell in the courthouse basement. In response to a note passed through the jailer by The Washington Post, the man told Montgomery Deputy Sheriff James Popp: "I'm innocent and she [his daughter] convicted me."

The daughter, a 15-year-old from Costa Rica whom the man and his wife had adopted in 1981, testified Tuesday that her adoptive father began forcing himself upon her during the first week that she and her brother were in the United States.

The man, whose name is not being used because it is shared by the girl and her brother, as well as by two of his natural daughters, began serving a 10-year sentence following his conviction in August. He is appealing that conviction.

He also faces charges of sexual abuse, rape, incest and unnatural and perverted acts brought by a third and fourth daughter who now are adults.

The jurors were not aware of those charges, nor of the man's earlier conviction, until Judge Calvin Sanders told them of the other cases after they had delivered their verdict.

According to testimony in the two trials and court papers, the man allegedly repeated a similar pattern of frequent and varied sexual activity with each child that spanned 13 years.

The jury of seven men and five women listened to testimony for 2 1/2 days this week, but reached its verdict after deliberating only an hour and five minutes.

The man's wife of 29 years, two of his natural daughters and his adopted son all testified during the trial. A neighbor of the family and several doctors were called by the defense.

But the most compelling testimony was delivered Tuesday by the 15-year-old, who wept repeatedly as she recounted details of the abuse that she said her adoptive father had subjected her to.

In closing arguments, prosecutor Barry Hamilton criticized the father's testimony Wednesday that the girl was lying. Hamilton called the father's testimony "absolutely mind-boggling triviality of a catalogue of [the child's] lies."

Most of what the defendant recounted involved the girl saying she had completed household chores when she had not. Hamilton said the evidence, and the testimony of other family members, suggested that the man lied, not the child.

But defense attorney Cleopatra Campbell of Frederick told the jury that "hysteria in this country about child abuse" had led the girl to concoct the story.

"Why would [the girl] do this?" Campbell asked the jury rhetorically. "Why did we have the witch hunts in Salem?"