The reproductive periodicity of the inland silverside, Menidia beryllina, was determined in laboratory experiments with two independent populations of fish. These populations were maintained in a non-tidal environment for 85 days and the daily number of eggs produced was enumerated. Egg production data were plotted in relationship to the equatorial tidal cycle (period 13.66 days) and the semilunar cycle (period 14.77 days). Numbers of eggs spawned were greatest on the day of occurrence of new and full moons during each semilunar cycle. These data indicate an endogenous reproductive cycle that can be maintained in the laboratory in the absence of tidal periodicity.