Five genera of invertebrates, collected from a municipal sewage lagoon, were fed to muskellunge (Esox masquinongy) fry for 23 days. The fry preferred Moina brachiata during both day and night (1300 and 2300 h) and Cyclops vernalis at night; they did not prefer Asplanchna sieboldi, Potamocypris sp., and Daphnia spp. Organisms in the foregut of fry collected at 2300 h were significantly larger, but not more numerous, than those in the foregut of fry collected at 1300 h. As the fry grew and the mouth width increased, the size of ingested organisms increased. Fry initially selected the first and second instars of M. brachiata and tended to avoid the later instars and adults; by day 23 they selected adults over immature instars. Immature and adult M. brachiata appeared to be of adequate size to feed muskellunge fry during alimentary canal development.