Sexual size and shape dimorphism was quantified in the whirligig beetle Dineutus nigrior Roberts. Females were larger in terms of body length, elytra length, body height, profemur width, and mass. Males had longer heads, longer and wider protarsi, longer and wider protibiae, and longer profemora. Discriminant function analysis identified 80.5% of individuals as the correct sex based on measurements of body size. Males and females also differed in terms of body shape. Potential causes of the dimorphism are discussed.