Mexican free-tailed bats are oft-unheralded saviors of South Texas. They prevent the region's crops from being overrun by pests, and the rest of us from being perpetually bothered by mosquitoes, gnats and other buzzy little beasts.
Watching them emerge from their daily slumber is also a fantastic and mesmerizing sight, and there are several places around San Antonio to do it. Within the city limits, tens of thousands of bats annually roost under I-35 near the intersection of Camden Avenue and Newell Street along the Museum Reach. The bats are there from roughly May through October, usually starting their ascent a little before dusk (though sometimes as late as 9:30 p.m.).
A bit further out of town near Natural Bridge Caverns, Bracken Cave houses over 20 million Mexican free-tailed bats — the world's largest colony. Check Bat Conservation International's website (batcon.org) for public viewing nights, which should be booked at least five days in advance.