This study examined the patterns of morphological variation both between species, and between sexes and among populations within each species of the Mexican sailfin mollies, Poecilia velifera and Poecilia petenensis, using geometric morphometrics and linear measure-ments of morphological traits. While sexes within each species differed in characteristics that may be important in sexual selection, such as length of the dorsal fin, species differed in traits, such as body depth, that may also be influenced by natural selection due to differences in habitats. Within each species, many morphological traits were similar among populations, but important differences, including caudal peduncle depth in P. petenensis (but not in P. velifera), suggested that habitat differences may also be important in shaping population divergence independently within each species. Indeed, the evolutionary vectors of male morphological population divergence for each species differed by an angle of 98Á5 , representing nearly orthogonal vectors and suggesting independent shape divergence between these two molly species. Finally, geographic isolation did not explain the morphological differentiation seen among populations, suggesting that natural and sexual selection were strong forces promoting morphological diversification within these two species, despite the potential for a high degree of population connectivity and gene flow.