Baby F, the one-pound boy who was the smallest of six living babies born to a California schoolteacher Tuesday, died early this morning of respiratory and heart failure, hospital officials said.

Nicknamed "Peanut," the 10 1/4-inch-long infant succumbed 64 hours after the birth of septuplets to Patricia Frustaci, 30, and her husband, Sam, 32. The seventh child, a 15.5-ounce girl, was born dead. The surviving three boys and two girls remain in critical but stable condition.

"We knew going in that Peanut was going to have to struggle," Sam Frustaci, an industrial steam equipment salesman, told reporters at Childrens Hospital of Orange County. Frustaci, fighting back tears, added: "Peanut surprised a lot of people. They didn't expect him to last 24 hours."

Although at least six sets of sextuplets are alive in the world, no more than five babies born to one woman at the same time have survived more than a few days in the United States.

The tiny Frustaci babies were born 12 weeks prematurely, and the largest of them weighed 1 pound, 13 ounces. They have battled many ailments common to severely premature infants, including poorly developed lungs, incomplete heart structures and jaundice caused by immature livers.

Doctors said today the heart problems seem to have responded to medication.

Thursday, reporters were shown a videotape of the father visiting Baby F in the neonatal intensive-care unit. "You keep fighting now," he said to the infant, who was lying on his back in a warming tray with tubes attached to his thin body.

Today, the unit's chief neonatologist, Dr. Carrie Worcester, said, "This baby was different from the other babies. This baby's umbilical cord was almost nonexistent" at birth. "Had this baby been in utero one more day, he would have been stillborn. This baby was a fighter."

Patricia Frustaci, who is recovering from the Caesarean birth of her seven children, is still isolated from the babies. She was only able to see Peanut after he had died, when she cradled him in her arms for about an hour, hospital officials said.

The Frustaci infants were given letters to identify them in order of birth. Baby F was delivered second to last in the three-minute Caesarean operation, before stillborn Baby G, whose umbilical cord was also very small.

Worcester said Baby F developed severe breathing difficulties at 7 p.m. Thursday but was revived after 40 minutes of intense effort by the large medical team supervising the infants. When the problem developed again several hours later, the baby could not be saved and died soon after midnight.

Hospital officials withheld announcement of the death until this morning's regular 9 a.m. news briefing.

"The hardest thing for Patti is she never got a chance to see him before he died , and that is a sad thing," Sam Frustaci said.

He said a combined funeral will be held for the two infants.

Officials said that Patricia Frustaci had planned to leave the intensive-care unit of St. Joseph Hospital, next door to Childrens Hospital, today and to visit the babies Saturday, but that Baby F's death may delay that schedule.

"It's a sad day," Sam Frustaci said, "but we have to be grateful we have five living . . . . We must concentrate now on the other five and hope and pray they will have a fighting chance."

All of the babies, conceived with the help of a fertility drug, are suffering from hyaline membrane disease, the lung condition that was the primary cause of Baby F's death, hospital officials said.

The infants' immature lung tissue lacks a substance that prevents air sacs from collapsing after each breath. They are being kept on oxygen and respirators.