The feeding ecology of the common smelt,Retropinna retropinna, was determined at several locations on the Waikato River system. The mean size of the dominant prey was found to increase as smelt mouth gape increased. Smelt smaller than 40 mm total length fed mostly on zooplankton. Chironomid larvae, pupae, and adults were the dominant foods of larger fish from most sites, although mysids and amphipods were the major prey in Lake Waahi and the Waikato River estuary. Algae were the dominant food in many smelt but ingestion was probably incidental. Seasonal and annual differences in dominant foods were minor. Common smelt are generalists, capable of feeding on the majority of smaller organisms present in their different environments; however, they may grow faster and achieve a larger adult size when food of optimal size and nutritive value is present.