Diabroticite beetles are economically important pests of North American agriculture. In the U. S., the most destructive of these beetles feed on maize, assorted cucurbits, and legumes. Many factors have contributed to renewed interest in their basic biology due to their invasiveness into new habitats and adaptability to management practices. For modeling and management, the sex of experimental insects generally needs to be determined, and is often associated with secondary sexual characteristics in males. We describe sexual dimorphism in external basitarsal structure of several corn rootworm beetles and the bean leaf beetle. We collected adult corn rootworms from maize and cucurbits in 2003-2004 and bean leaf beetles from soybeans in 2001-2004. Sex of each of 100 adult specimens per test species was determined using both the structure of basitarsi on legs separated from the thorax and from abdominal morphology using the presence or absence of the supra-anal plate. The tarsal sexual dimorphism in question comprises the presence in males but not females of hairless planar patches on the proximal ventral surface of the basitarsus or first tarsomere. This specialization occurs on prothoracic and mesothoracic legs of Diabrotica males but only on the prothoracic legs of the bean leaf beetle. Even in the Diabrotica species examined, however, the patches are somewhat larger on the prothoracic than the mesothoracic legs. We relate this sexual dimorphism in tarsal morphology to mating behavior and the propensity for diabroticite males to retain tarsal contact with the elytra of females during copulation.