Snout-vent length of 659 specimens of Microhyla olivacea and M. carolinensis from Texas, Oklahoma, Louisiana and Florida was compared in respect to the overlap zone of the two species. The former species decreases in size from west to east; the latter increases from east to west. The greatest size differences between the two species occur in the zone of overlap in eastern Texas and Oklahoma. Size difference is considered a partial isolation mechanism, which may be undergoing reinforcement through selection in the zone of overlap and limited hybridization.