When city officials finished reworking the concrete and steel Congress Avenue Bridge four years ago, they boasted of its I-beam permanence and its five-lane prominence over Town Lake leading into the heart of downtown. But they forgot about the bats -- the hundreds of thousands of Mexican free-tailed bats that migrate seasonally between southern Mexico and the northern part of the hemisphere.

Since the bridge reopened, the bats have taken over its nooks and crannies as hotels. "That bridge could not have been designed better as a bat cave," said Dr. Phil Zbylot, a city health administrator.

At dusk on any calm night, swirling colonies of the finger-long bats drop from under the bridge and fly east to gorge on insects that hover over Town Lake and other nearby lakes fed by the Colorado River. They return by dawn, screeching like busted bicycles as they sail into "bed" over the heads of joggers on the hiking trail that borders the lake.

Throughout the Southwest, zoologists say, about 30 species of migratory bats check in for rest and recreation each August and September and check out for warmer climes before the first winter freeze. They visit again in the spring, separated into nursery and bachelor colonies, before most of them head farther north.

Though 2 to 3 percent of the bats carry rabies, they usually flit on by and rarely bite. Moreover, Austin is long since accustomed to dealing with thumb-sized cockroaches, bedroom mosquitoes and crickets that breed like locusts along the 14 blocks of Congress Avenue past the bridge and the new skyscrapers to the pink granite Capitol.

"We have more people getting drunk and hurt in other ways than being bit by bats," hospital clerk Jamie McWha said.

Nonetheless, at least four people have been bitten this year. In August, Cyndi Hughes, an editor for the Austin American-Statesman, found a scratch on her hand after a bat got tangled in her skirt outside the newspaper building, a few hundred yards from the infested bridge. Hughes has had to undergo a series of rabies shots. "I don't foam at the mouth anymore and I'm rabies-proof," she said, exhibiting a gift rubber bat pen.

Aside from leaving behind musty dung, the bats each day devour several times their weight in insects, particularly mosquitoes and moths, says Charles Sweet, director of the state health department laboratory.

"By far, there's more of a problem with crickets than bats," said Sweet. During a 1983 University of Texas football game, Sweet recalled, he sat in the stadium's upper deck and ended up with nearly 100 lusty crickets caught between two pieces of rain gear.

Bats, crickets and unwanted pigeons have always been a problem at the 75,000-seat stadium. A few years back, university administrators paid $25,000 to seal the cracks in an overhanging deck, from which bats threatened the 8,000 spectators below. And until this year, the state certified an Austin exterminator to kill bats by spraying cyanide gas under the stands.

"If these bats weren't here, we'd probably be spraying the lake with pesticides," said Dick Robbins, president of a Dallas firm that manages the Sheraton Crest Inn adjoining the bridge. Roughly 150 rooms have views of the nightly bat clouds as they flutter from under the bridge. Robbins jokes that he has considered charging patrons $5 to catch the ritual.

As for the Congress Avenue bridge, city officials have considered plugging its concrete bat "caves" with wire screens, hardware cloth and even polyurethane rubber, said Zbylot.

"But where are they going to go to then?" he said. "They may well prefer other sites in the downtown area. The parking garages. That would pose a much bigger problem. We would have to look at exterminating. And that would be too drastic."

Zbylot noted that from 1937 to 1970, Texas built dams diverting the flow of the Colorado River into nine manmade lakes and obliterating hundreds, if not thousands, of bat caverns.

"We have occupied some of their housing," Zbylot theorized. "A shift of 250 miles is nothing to them."