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Bracken Cave, a large natural cavern located 25 miles northeast of San Antonio, Texas, serves as nursery roost for millions of Mexican free-tailed bats (Tadarida brasiliensis). Ten days after spending several hours in the cave in early May 1979, three graduate students presented with signs and symptoms compatible with acute pulmonary histoplasmosis. A study was initiated to determine the presence and location of Histoplasma capsulatum in the cave and its inhabitants. The fungus was cultured directly from guano at five of nine sites sampled. Sentinel mice left for several hours at one of three positive sites developed histoplasmosis within 30 days after exposure. Autopsy was done on a total of 28 bats and their lungs, liver and spleen were cultured for viable H. capsulatum. In addition, sera from 58 bats were tested for antibodies to H. capsulatum, using a double diffusion test employing commercial reagents. All bat cultures and sera were negative. We conclude that the students were infected by massive exposure to viable H. capsulatum in aerosols created by disturbing guano deposits near the mouth of the cave. Tadarida brasiliensis contributes to a suitable environment for fungal proliferation, but does not appear to play an active role in the maintenance or dissemination of the fungus in Bracken Cave.