The federally endangered Myotis grisescens (Gray Bat) is a cave-obligate species that exhibits swarming behavior outside hibernacula in the fall and then emerges in spring after hibernation. Sex ratios of other Myotis are documented to shift at hibernacula from male- to female-dominant in the fall swarming period, and from female- to male-dominant in the spring emergence. We captured Gray Bats during the winter of 2013, fall of 2013 and 2014, and the spring of 2014. Using chi-squared analysis to compare differences among seasons, we document changes in sex ratio for a Gray Bat population at a single large hibernaculum. The sex ratio shifted from male- to female-dominated in the fall of 2013 and 2014 and again in the spring of 2014. During the winter, while Gray Bats are hibernating, we documented a 1:1 ratio. Based on our results, it appears that Gray Bats may exhibit different behavior compared to other Myotis in the spring, possibly linked to being cave obligates.