ASYMMETRY of the abdominal segments in the corixid males is a regular occurrence with the sense of asymmetry, dextral or sinistral, regularly uniform intraspecifically. One finds among the males no intermediate forms, for all the asymmetrical structures of a dextral male are perfect mirror-images of those in a sinistral male and vice versa. More particularly, the sex-limited structures are confined to the body wall, and more precisely, to the last six segments where the exoskeleton and musculature display an asymmetrical pattern, emphasized by an oval black body (the strigil) and a curved genital capsule. In the dextral male the strigil is limited to the right margin of the sixth segment and the genital capsule is directed to the right. These asymmetrical structures are, of course, mirror-images of those in a sinistral male, while in the females bilateral symmetry prevails. Instances of situs inversus 1 are rare.