Amongst the bivalve molluscs, the reproduction of the Chilean oyster,Tiostrea chilensis(Philippi, 1845), is characterised by several unusual features including very low fertility, extremely large eggs and larvae, an extensive period of brooding, and an almost non-existent planktonic larval stage with very limited dispersal. Despite these features being well-described, the gametogenic cycle of this oyster remains poorly understood. It might be expected that the gametogenic cycle ofTiostrea chilensiswould follow the trend toward increasingly labile sexuality that is seen among oyster species with less extensive brooding behaviour. Consequently, the gametogenic cycle in a population of Chilean oysters from north-eastem New Zealand was studied over 20 months from April 1994. Histological preparations revealed that the oysters are protandrous, maturing first as males by about 16 mm in size. From approximately 31 mm some oysters begin to produce ova and oysters over this size are mostly simultaneous hermaphrodites. Eggs and spermatozoa are spawned all year round, although there is a peak of male spawning in late summer-autumn. In contrast to other species of hermaphrodite oysters that brood their larvae, there is no evidence to support the presence of regular alternation of sexes in this population ofTiostrea chilensis.Ripe ova and spermatozoa ark frequently found together in the follicles and are apparently spawned simultaneously suggesting that self-fertilisation is possible (previously true functional hermaphroditism was thought to be very unlikely in the Ostreidae). Overall, the gametogenic cycle in this population ofTiostrea chilensisis dominated by the production and release of spermatozoa in the form of spermatozeugmata. The predominance of spermatogenesis, hermaphroditism and the presence of spermatozeugmata are discussed in relation to the potential difficulty experienced by this oyster in fertilising broods of eggs.