Irruptions of gizzard shad, Dorosoma cepedianum (Le Sueur), in North Twin Lake, Iowa in 1954 and 1955 provided opportunities for investigating certain basic ecological relationships of this species. Of the approximately 358 plankton species and varieties found in North Twin Lake, 260 were encountered in shad digestive tracts. On a volume basis, phytoplankton (primarily Cyanophyta and Chlorophyta) constituted the bulk of young-of-the-year and yearling gizzard shad food. Crustacean zooplankton were important food elements of recently hatched individuals.