[Summary]. Cymodetta gambosa, a new genus and species of the isopod family Sphaeromatidae, is described from specimens in a culture derived from the Clarence River at South Grafton, New South Wales, Australia. The new genus belongs to Hansen's Hemibranchiatae, and apparently to his section Sphaeromini, but in contrast to known genera of Sphaeromini has uniramous uropods. The 5th pereopods exhibit a striking sexual dimorphism; in the male the propus is expanded into a prominent lobe overriding the dactyl. Mating behaviour is described, including the function of the propal lobe in preventing the female from rolling up.