Body mass is commonly used as an indicator of habitat quality and body condition in small-mammal populations. We examined the effects of consecutive days of livetrapping on body mass of 2 federally listed subspecies of beach mice on the eastern coast of Florida: the Anastasia Island beach mouse (Peromyscus polionotus phasma) and the southeastern beach mouse (P. p. niveiventris). Mean mass loss (approximately 0.5 g/recaptured day) was additive and occurred for Anastasia Island beach mice over 2 consecutive days and for southeastern beach mice over 3 consecutive days. Researchers should exercise caution when their trapping protocols call for consecutive nights of trapping.