SCHENECTADY, N.Y., JULY 17 -- Marybeth Tinning, whose nine children have died one by one over the last 15 years, was convicted today of smothering the ninth, 17-week-old Tami Lynne, a few days before Christmas 1985.

Schenectady District Attorney John B. Poersch said he plans to prosecute Tinning in the deaths of at least two of the other eight children. "I can assure you that this is Round 1," he said.

Tinning, 44, a former school bus driver, sobbed quietly as the verdict was announced, after 19 hours of jury deliberations. Her husband, Joseph, said later, "I still think she's innocent."

Tinning was found not guilty of intending to kill the child, but guilty of "acting recklessly with a depraved indifference to human life." The minimum sentence is 15 years in prison. Defense attorney Paul M. Callahan said he would appeal.

The first of Tinning's children to die, 8-day-old Jennifer, died in 1972 of acute meningitis before she left the hospital where she was born. An anonymous phone call to authorities after Tami Lynne died prompted an investigation of her death and the deaths of the seven other Tinning children, one of them adopted.

Poersch relied heavily during the four-week trial on Tinning's statements to police when they interrogated her in February 1986. Tinning described placing a pillow over Tami Lynne's head to quiet the wailing infant.

"I didn't mean to hurt her," the statement quotes her as saying. "I just wanted her to stop crying."

The prosecution never presented a motive for the killing, but emphasized Tinning's statement that she felt she "wasn't a good mother."

One handwritten page of the statement said, "I did not do anything to Jennifer, Joseph, Barbara, Michael, Mary Frances, Jonathan. Just these three, Timothy, Nathan {her fourth and fifth children to die} and Tami. I smothered them each with a pillow because I'm not a good mother. I'm not a good mother because of the other children."

William Barnes, now a retired state police investigator and the first to take Tinning's statement, has known Tinning since she was 10 years old. He said in an interview today he thought the attention showered on her after the first child's death may have prompted her to kill the others.

Tinning told Barnes that her father had locked her in her room and beaten her as a child, Barnes said. Although the information was not used at trial, she also said she once tried to poison her husband.

Tinning did not testify and has refused to comment. But in a pretrial hearing last December, she testified that she submitted to the police interview because detectives threatened to "take my children from their graves and rip them limb from limb."

Callahan, the defense lawyer, sought to show that police failed to advise Tinning properly of her rights to remain silent and have an attorney present.

The lawyer also attempted to prove that Tami Lynne died from a rare genetic disorder, Werdnig-Hoffmann disease, described by doctors as the childhood counterpart to Lou Gehrig's disease, amyotrophic lateral sclerosis.

But two key prosecution witnesses -- Dr. Marie Valdes-Dapena of Miami, president of the SIDS (Sudden Infant Death Syndrome) Foundation and Dr. Thomas Oram, the medical examiner who conducted the autopsy -- said they believed that Tami Lynne was smothered with a soft object.

Dr. Bradley Ford, the baby's pediatrician, testified for the prosecution that Tinning had rejected his suggestion that, in view of the other children's deaths, she should use a special alarm device to monitor Tami Lynne's breathing and heart rate.