The information available on diversity of mammals in Natural Protected Areas of Yucatan, Mexico has been poorly documented. Between October 2001 and March 2002 we sampled bats and small rodents in three vegetal associations from Ría Celestún Biosphere Reserve. In total we captured 12 species: seven bat species being Dermanura phaeotis the most abundant in the dry forest, and five small rodent species were found being Peromyscus yucatanicus more abuntant into hummock forest (peten). The habitat with highest specific richness of bats was the dry forest (H’=1.39) while peten was to small rodents (H’=0.838). Finally we made the second report of Chiroderma villosum in Yucatan state.