This report documents the results of a study of bat occurrence in the Pryor Mountains of south central Montana that was initiated in 1989 (Worthington and Ross 1990). The 1990 field research was conducted from 15 June to 14 September and resulted in a total capture of 1,101 individuals of 10 species. Bats were captured at ponds, springs, and at the entrance of five caves. Numbers of bats captured were generally greater at the caves. Capture success was considerably lower at water sources, but a greater diversity of species was noted at these sites. While several of the species captured occurred throughout the area, the spotted bat, Euderma maculatum, the pallid bat, Antrozous pallidus, and the silver-haired bat, Lasionycteris noctivagans, were more restricted in distribution. Two specimens of the spotted bat, Euderma maculatum, were captured. These specimens represent the first live spotted bats captured in Montana since 1949 (Nicholson, 1950; Hoffmann and Pattie, 1968). Spotted bats were observed throughout the eastern portion of the study area. The Townsend Big-eared bat, Plecotus townsendii, was previously known only from winter record in the Pryor Mountains; 11 individuals were observed in 1990. Seventeen individuals of Antrozous pallidus were captured, bringing to 26 the total number captured in the area since the species was first documented in Montana in 1979 (Shryer and Flath, 1980; Worthington and Ross, 1990). Several of the species of bats found in the Pryor Mountains were captured in numbers significantly different from an expected 1:1 sex ratio. This was especially true at the caves, where males greatly out-numbered females, suggesting that in some species males and females may be differentially utilizing habitat. The generally low temperatures of the caves investigated in this study may preclude their summer use by many female bats, especially pregnant or lactating individuals, which require higher roost temperatures in order to maintain the higher metabolic rate necessary for raising young (Racy, 1982a). Bat activity at the caves, especially Mystery Cave, indicated that these caves provide important summer roosting habitat. Additionally, these caves possess characteristics which may make them important as hibernacula.