GAINESVILLE, Fla. (AP) _ Two decades ago, experts declared the Florida Atala extinct, but now it is one of three species of butterfly making comebacks after nearly disappearing.

Along with the Schaus Swallowtail, the iridescent blue-black Florida Atala exists only in the southern part of the state, while the Sweadner's Hairstreak is found only around St. Augustine.

Officials thought the Florida Atala was extinct in the 1960s, ''but apparently a small population hung on in Key Biscayne State Park,'' said University of Florida zoology professor Thomas C. Emmel.

The insect feeds on coontie, a palm-like plant that once was common in the state but has virtually disappeared. Emmel said the Atala apparently survived by adapting to feeding on ornamental cycads, such as sago palms, planted by area residents.

''From a tiny nucleus of butterflies in Key Biscayne, it came back and now it ranges out across the mainland,'' Emmel said.

Emmel said the Schaus Swallowtail's return is similiar.

In 1984, Emmel and students surveyed Elliott Key, parts of Key Largo and some of the other smaller keys in Biscayne Bay for the showy, large yellow and brown Swallowtails.

At the time, he estimated the total population was less than 70. Only one butterfly was spotted on Key Largo.

''I thought with a population that low, it probably would become extinct, and we were considering a captive breeding program,'' he said.

Emmel says four factors influenced the decline of the Schaus Swallowtail: pesticides, hurricanes washing over the low-lying keys, capture by butterfly collectors and habitat clearing.

But in 1984, Emmel found 40 Swallowtails on Key Largo alone, and this year he found 70 on the key. Its overall population had increased to 1,000.

''The butterfly is making a remarkable comeback,'' he said. He said its resurgence on Key Largo may be due partly to an end to mosquito spraying on the northern part of the key, and to ''the sheer happenstance of construction being halted on Key Largo because of a water shortage.''

The fate of the Sweadner's Hairstreak is less certain.

It lives in a small area near the St. Augustine lighthouse and Anastasia State Park where some red cedars, its food source, grow.

Two stands of cedars, one at the lighthouse and one at the park, sustain the butterfly.

''Today, the butterfly hangs on by a toenail, but as long as those cedars aren't cleared, it's got a pretty good chance,'' said Emmel.

''It's possible for even a critically endangered species to spring back if people are sensitive to its environment,'' Emmel said.