The Caenorhabditis elegans sex determination gene tra-3 is required for the correct sexual development of the soma and germ line in hermaphrodites, while being fully dispensable in males. Genetic analysis of tra-3 has suggested that its product may act as a potentiator of another sex determination gene, tra-2. Molecular analysis reported here reveals that the predicted tra-3 gene product is a member of the calpain family of calcium-regulated cytosolic proteases, though it lacks the calcium binding regulatory domain. Calpains are regulatory processing proteases, exhibiting marked substrate specificity, and mutations in the p94 isoform underlie the human hereditary condition limb-girdle muscular dystrophy type 2A. The molecular identity of TRA-3 is consistent with previous genetic analysis which suggested that tra-3 plays a very selective modulatory role and is required in very small amounts. Based on these observations and new genetic data, we suggest a refinement of the position of tra-3 within the sex determination cascade and discuss possible mechanisms of action for the TRA-3 protein.