We measured 140 stripe-necked terrapins (Mauremys leprosa) from the center of the Iberian Peninsula, Spain. Female body size exceeded that of males, and female shells were wider and more domed than those of males in relation to carapace length. Plastron length was also greater in females, but preanal tail length was greater in males. Tail length showed similar values in both sexes. Morphological aspects related to sexual selection processes and those related to the fecundity of females are discussed as possible explanations of the sexually dimorphic traits in this species.