Embryo-free, fat-free live body weight and lean dry body weight showed significant differences in all reproductive seasons (prepregnant. pregnant, lactating, post lactating) for Myotis thysanodes while only prepregnant M. lucifugus had significantly lower fat-free live and lean dry body weight than at other reproductive states. Prepregnant and post lactation wing loadings in M. thysanodes were significantly lower than during pregnancy and lactation whereas wing loadings for M. lucifugus followed the pattern for body weight. Also, maximum wing loading was significantly greater than minimum during all reproductive seasons for M. thysanodes but only for prepregnant and pregnant M. lucifugus, thus indicating significant daily decreases in wing loading at these times. Fat indices for both species were consistently low until fall at which time a significant increase occurred. Water index and gross body composition (percent water, percent non-fat organic, and percent mineral) followed the same pattern as body weight for each species. The left adrenal was significantly heavier than the right in M. thysanodes and both were significantly heavier during lactation than at any other time. A significant increase in spleen weight from prepregnant to post lactating M. thysanodes occurred, thus indicating an increase in erythropoietic activity at this time. No significant changes were observed for organ weight in M. lucifugus. Thus, the two species examined showed a number of differences which may relate to differences in their physiological-ecological strategies.