Sex ratios of Myotis velifer populations were studied during 2 hibernating seasons within and among 42 caves across a portion of its geographic range in Oklahoma and Kansas. The number of females to males was significantly different than the expected 1∶1 sex ratio in 25 of 44 visits to hibernacula. The percentage of females at the Selman Cave System, one of the largest hibernacula in Oklahoma, was always greater than the percentage of males throughout the season and in clusters of different sizes. Neither females nor males were any more likely to be found at the center than on the periphery of large clusters. The sex ratio of volant young at a maternity roost did not differ significantly from the expected 1∶1 sex ratio.