Reproduction in Fundulus heteroclitus is adapted to the temperate climate and the tidal environment of the coastal marshes which are its primary habitat. Egg deposition and spawning are closely linked to the tidal cycle. Concentration of spawning on spring tides gives the fish access to sites in the high intertidal zone. Eggs are deposited near the high water mark, usually in sand in New England populations and in Spartina alterniflora or empty Geukensia demissa shells in Middle Atlantic and southern populations. Aerial incubation of eggs appears to be essential for their survival in the salt marsh. Loss of eggs due to lack of fertilization, mortality during development and predation was always less than 30 percent in our observations. Hatching occurs only when the eggs are inundated, usually on spring tides. An endogenous semilunar gonadal cycle appears to be involved in control of spawning, but an effective environmental synchronizer has not been found. Cyclic changes in estradiol-17β accompany the gonadal cycle, while spawning behavior is stimulated by neurohypophyseal hormones. Both temperature and photoperiod have been implicated in the control of seasonal reproduction in F. heteroclitus. Early workers focused on males and concluded that increasing temperature was the stimulus that initiated gonadal recrudescence in spring. These experiments were not rigorous tests of the effects of photoperiod. We have shown that in females both warm temperatures and long photoperiods are involved. In female F. heteroclitus, as in most mammals and birds, it is the timing rather than the duration of the light exposure which determines its effectiveness in stimulating gonadal maturation. Maintenance of gonadal maturity in female F. heteroclitus depends on long photoperiods, but the ovary may become refractory and regress even when stimulatory photoperiods are sustained in the laboratory. Ovarian maturity can be stimulated with gonadotropin injection in refractory animals, indicating that the brain-pituitary axis is the site of refractoriness. The pathway by which light influences ovarian maturity may involve an encephalic photoreceptor, since neither the eyes nor the pineal gland is necessary.