We examined seasonal variation in urine concentration and fecal water content (FWC) of free-ranging bats inhabiting the tropical deciduous forest of the Chamela-Cuixmala Biosphere Reserve, Mexico, which is characterized by marked dry and rainy seasons. We tested the hypothesis that bats inhabiting this forest would exhibit seasonal physiological changes to maintain water balance. We predicted that bats would have higher urine concentrations and reduced FWC during the dry season. Our results were consistent with the 1st prediction for the insectivore, Parnell's mustached bat (Pteronotus parnellii), but not for the Jamaican fruit bat (Artibeus jamaicensis). Our data were consistent with our 2nd prediction for P. parnellii and the facultative nectarivore Pallas' long-tongued bat (Glossophaga soricina), but not for A. jamaicensis. Our findings indicate that for the bats we studied, the 2 routes by which water is lost were affected differently by seasonal changes in water availability as a function of feeding habits. © 2010 American Society of Mammalogists.