Ontogenetic changes in body shape and its associated allometry were studied in the Blacktail Shiner, Cyprinella venusta, using geometric morphometric methods. We used a single, large sample (n = 397; 182 males, 215 females), collected in Catahoula Creek, Jourdan River drainage, Hancock County, Mississippi. Ten body landmarks were digitized from each specimen, which yielded partial warp scores that were used as shape variables to describe body shape change during ontogeny, assess sexual dimorphism, and investigate the relationship between reproductive status and ontogenetic body shape change. We also assessed the effect of sexual dimorphism on size and body shape. The null hypothesis of isometry during ontogeny was strongly rejected by multivariate regression of shape on size for both sexes (males, P < 0.0001, F = 21.970; females, P < 0.0001, F = 16.238). We found large, highly significant sexually dimorphic differences in the body shapes of males and females (MANOVA for overall shape, P < 0.0001, F = 7.535, Wilks' lambda, 0.758), which remained significant using MANCOVA with size as a covariate (log SL, P < 0.0001, F = 34.872, Wilks' lambda, 0.438; log CS, P < 0.0001, F = 34.829, Wilks' lambda, 0.439). Moreover, the ontogeny of body shape differs between males and females. There were highly significant shape differences among reproductive classes within males and females. These findings suggest that change in reproductive status may occur in concert with body shape change.