This study characterized loss in body mass by the cave myotis, Myotis velifer, in 7 hibernation seasons (October–March 1979–1986) in western Oklahoma. Average mass entering hibernation was 14.4 g for males and 15.4 g for females. At the end of hibernation, average mass of males and females was 11.5 and 12.0 g, respectively. Males lost an average of 2.9 g (20.1%) and females lost 3.4 g (22.1%). During hibernation, males and females lost 0.021 and 0.024 g/day, respectively. We detected no difference in rate of loss of mass between the first and second halves of hibernation for either sex. Males and females lost 3.8 and 4.1 g, respectively, during 2005–2006, when only two visits were made to the cave (October and March). These values for loss in mass were slightly more than losses recorded for males and females when bats were sampled each month during hibernation.