Anatomical changes to the gonad during sex change in the protogynous grouper Epinephelus rivulatus are described from histological observations. A decrease in ovarian mass occurred soon after the onset of sex change as oocytes atrophied and were removed from the gonad. Blood supply and abundance of unidentified somatic cells increased at this time as proliferation of sperm tissue commenced. As the gonad was cleared of ovarian tissue, the rate of spermatogenesis increased and the lamellae soon became dominated by sperm and connective tissue. Putative Leydig cells, the probable sites of male steroid production, appeared in transitional gonads and were most abundant in the testes of immature males. Peripheral sperm sinuses subsequently formed within basal tissue layers of the tunica and expanded as they filled with spermatids. The process of sex change, occurring as a result of experimental manipulation of wild populations at the start of the spawning season, took c. 3 weeks. This appears rapid compared to other hermaphroditic species and may reduce the impacts of fishing on reproductive output by E. rivulatus populations.