Our present understanding of the structure of the gonads and of gametogenesis in Fundulus helerodilus is briefly reviewed. The testes contain spermatogenic tubules which distally are composed of linearly arranged germinal cysts with clones of synchronously developing germ cells, and proximally of efferent ducts with mature sperm. Within the tubules, those cysts containing spermatogonia are found close to the testicular periphery, whereas cysts with mature sperm are contiguous with the efferent ducts. Cytological details of the three principal stages of spermatogenesis, namely spermatocytogenesis, meiosis, and spermiogenesis, are presented along with examples of the various somatic cells of the testis. The ovary consists of numerous ovigerous lamellae which contain randomly arranged follicles in various stages of development. Since follicular growth is asynchronous, follicles of all sizes are present in the ovary during the breeding season and eggs are continuously ovulated into the ovarian lumen. Oocyte growth is divided into five distinct stages: Primary growth (substages A and B), cortical alveolus formation, vitellogenesis, oocyte maturation (substages A and B), and the ovulated egg. This series of stages is based both on cytological observations and physiological events.