A 20-month-old boy was in critical condition at Children's Hospital yesterday after he fell into a swimming pool at his baby sitter's home in Glenn Dale, Prince George's County police said.

Antonio Simms of Bowie was found floating in the pool about 10 a.m., police said, after his baby sitter noticed that one of the dozen youngsters in her charge was missing.

Cpl. Bruce Gentile, a county police spokesman, said that the in-ground pool has a fence around it. Gentile said police do not know how Antone got inside the fence.

Police are calling the incident an accident.

Twelve youngsters, from infants to toddlers, were being cared for in the private home at 10412 Cleary La.

The baby sitter was not licensed to operate a day care center, police said.

A county ordinance requires people who care for at least five children at least twice a week to be licensed.

Police said that their investigation is continuing and no charges have been filed against the baby sitter.

Police said the baby sitter, whose name was not released, pulled Antonio from the pool, began administering cardiopulminary resuscitation, and called the county emergency number for a rescue squad.

The emergency workers helped to restore a pulse, police said, and took him to the Prince George's Hospital Center. He was later taken by helicopter to Children's Hospital.

Yesterday's near-drowning was the second incident since March in which a child in the care of an unlicensed baby sitter was injured.

On March 25, a 2-year-old boy was killed and a 2-year-old girl was injured after their baby sitter, Angelia Maria Greene, 31, of New Carrollton, allegedly beat them after becoming angry when they wet their clothing.

Greene, of 7606 Fontainebleau Dr., was charged with first-degree murder in the beating death of Brandon White, of 4231 58th Ave., Bladensburg. The case has not yet gone to trial.