Gonad examinations were made on billfish caught during a tuna longline survey in the equatorial western Indian Ocean. The species considered were Istiophorus platypterus, Tetrapterus audax, Tetrapterus angustirostris, Makaira nigricans, and Makaira irtdica. The morphology of the urino-genital system is described for all species. It was found to be similar in all except T. angustirostris in which the gonads are Y-shaped rather than bilaterally symmetrical. The histology of the gonads is described, and the changes within the testis and ovary are traced throughout the spermatogenic and oogenic cycles. These are shown to be similar in all species examined.