Quantifying overwinter mass loss in bats is important for understanding hibernation energetics, habitat conservation, and the ability of bats to persist with novel pathogens. Townsend's big-eared bat (Corynorhinus townsendii) is a species of conservation concern. Little is known about overwinter mass loss of this bat in western North America. We conducted a retrospective analysis to quantify overwinter mass loss of 362 females and males during 1987 and 1988 in 5 caves in an important area for the conservation of this bat in western North America. Although body mass of 13 females in cave C62 averaged 1.1 g heavier than 12 females in cave C54 when captured in October, all those females recaptured in March lost similar percentages of body mass (x̄ = 22.6%) over winter. In cave C27, body mass of 6 females averaged 2.6 g heavier than 8 males when captured in October. Those females and males recaptured in March lost similar percentages of body mass (x̄ = 19.1%) over winter. In caves C27 and C54, mass of 96 male bats decreased by a mean of 21.1% between October and March. Across all five study caves, mass of 227 female bats decreased by a mean of 23.7% between October and March. Our results indicate that females are heavier than males when entering hibernation, and that females generally lose more mass than males during hibernation. Moreover, female bats entering hibernation in our study area weighed more than female bats of this species in other studies when entering hibernation. Our data provide researchers in western North America with mass loss estimates for female and male Townsend's big-eared bats.