The human Rhesus (Rh) family consists of three polytopic membrane proteins present at the cell surface of red blood cells. Although Rh proteins are essential for the expression of the blood group system their biological function remains unclear. In this study, the gene encoding a protein homologous to Rh50 in Dictyostelium discoideum was sequenced. The Rh50-like protein was localized to the contractile vacuole, the organelle responsible for maintenance of osmotic equilibrium within the cell. However, Rh50-like-deficient mutants in which the Rh50-like gene was disrupted did not appear to exhibit a phenotype related to osmoregulation. Nevertheless, these mutants may provide a valuable tool for studying the function of the rhesus protein.