The majority of juvenile alewives (Alosa pseudoharengus) migrating seaward from Hamilton Reservoir, North Kingstown, Rhode Island, did so on less than 10 per cent of the total days of their period of residence in freshwater during 1970, 1971, and 1972. Extensive migration occurred following increases in pond outflow, abrupt water temperature declines, and/or precipitation. Daily patterns of movement were unimodal, with maximum movement occurring in mid-afternoon. Groups of fish migrating downstream during periods of extensive migration were random portions of the pond population. Growth rates varied in different parts of the drainage system. A model of the behavioral basis for seaward migration of juvenile alewives from ponds and lakes is presented.