The reproduction ecology and spawning periodicity of the Atlantic silverside, Menidia menidia, living in the North Edisto River estuary, South Carolina, was studied at two sites, Bears Bluff and the Point of Pines, during the spring and summer of 1976, 1977, and 1978. A specific sequence of behavioral phenomena, lasting for 30 to 60 minutes, preceded a spawning run in schooling M. menidia. Spawning rungs occurred only on daytime high tides and were precisely timed to coincide with the predicted time of high tide. Spawning locations along the shoreline at the two study sites were not evenly distributed. Spawning by large numbers of fish in a small area at high tide, when current velocities were low, resulted in dissolved oxygen depletion to measured concentrations of <1.0 mg/l.