Abstract. Histoplasma capsulatum was isolated from gut, lung, liver, and spleen of 17 of 208 captured bats belonging to 6 different genera and species. Three of the 17 infected bats were from the State of Guerrero and 14 were from the State of Morelos. All were adult bats: 6 males (1 Pteronotus parnellii, 2 Natalus stramineus, 2 Artibeus hirsutus, and 1 Leptonycteris nivalis) and 11 females (1 Myotis californicus, 1 Mormoops megalophylla, 8 A. hirsutus, and 1 L. nivalis). High rates of bat infection with H. capsulatum were found in the monitored sites of the State of Morelos. Histoplasma infection of N. stramineus, A. hirsutus, and L. nivalis should be considered as the first records in the world. The fungus isolated from infected bats was identified by its typical mycelial-phase morphology and by its yeast-phase conversion. Exoantigen production confirmed the fungal identification by the presence of specific precipitation lines in double immunodiffusion assays using human immune serum. Histopathologic studies showed intracellular yeast-like cells compatible with H. capsulatum yeast-phase in tissues of several bats, especially in pulmonary (intra-alveolar and septal) macrophages, with none or minimal tissue reaction. In contrast to past reports, present data support a high risk of bat infection with H. capsulatum in Mexican cave environments.Bat guano and bird droppings have been shown to be the most common sources of the pathogenic fungus Histoplasma capsulatum var. capsulatum Darling, 1906, the causative agent of the deep mycosis histoplasmosis, which has a worldwide distribution.1 The fungus is found either in confined spaces where bat guano is abundant or in open spaces such as public parks and home yards, where bird droppings are frequently found. Excreta from these animals are rich in nutrients necessary for fungal growth and together with soil and environmental conditions, humidity and temperature, constitute the ecologic niche of this microorganism. [1][2][3][4] Bats are among the few infected mammals that contribute to the maintenance of this fungus in natural foci, in addition to some gregarious birds such as starlings, black birds, chickens, oil birds, and pigeons. Aguirre-Pequeño in 1959 5 and González-Ochoa in 1963, 6 defined the habitat of bats related to the isolation of H. capsulatum in Mexico, and the latter tested experimental infection of Desmodus rotundus in the laboratory without success. Fungal infection of Myotis californicus and Mormoops megalophylla is considered the first record in the world, whereas P. parnellii infection is new for Mexico. Although the environmental factors promoting H. capsulatum growth in shared ecologic niches have long been known, the circumstances that favor bat infection have been poorly studied. This paper presents data that might contribute to the understanding of the conditions that foster bat infection with this fungus. MATERIALS AND METHODSBats capture sites. Bat specimens were captured from 11 different sites located in 2 states in Mexico: Guerrero and M...