The reproductive periodicity of the Atlantic silverside Menidia menidia was studied at two locations on the North Edisto River estuary in South Carolina during March-July 1976–1978. Spawning runs occurred in the upper intertidal zone and coincided precisely with daytime high tides. Time-series analysis of daily changes in the intensity of spawning runs revealed a fortnightly reproductive periodicity and indicated that the observed reproductive rhythmicity in Atlantic silversides may be mediated by a high-tide-sunrise cue that also occurs at fortnightly intervals. During the 1976 and 1977 reproductive seasons, there were highly significant correlations (P < 0.01) among the male gonadal index, the female gonadal index, and the occurrence of intermediate, maturing, and hydrated-egg stages of sexual development in females. The percentage of females with hydrated eggs was greatest on days when a high tide occurred within 1 hour after sunrise.