Young gizzard shadDorosoma cepedianumcan have dramatic effects on ecosystem structure because their high consumptive demand can eliminate crustacean zooplankton populations. The objective of this study was to examine diel feeding chronology of age-0 gizzard shad (25–89 mm total length), as well as their daily ration and consumption ofDaphniaspp., in Oneida Lake during 1991 and 1992. Gastric evacuation titres, treasured experimentally, were faster for fish fed continuously (0.8–1.8 h) than for fish fed single meals of zooplankton (4.0–7.2 h) and were influenced by fish size. In Oneida Lake, young gizzard shad exhibited a diel feeding cycle; gut fullness gradually increased to a maximum during 1100–1900 hours and declined during the night. Juvenile gizzard shad shifted from a diet of exclusively zooplankton in July to a combination of zooplankton, algae, and detritus in August. Estimates of total daily ration ranged from 1.2 to 21.3% of wet body weight, and total daily ration of young gizzard shad increased with fish size. Our estimates of daily ration, based on fish feeding continuously during the day and digesting a single meal at night, closely approximated the maximum daily field ration of age-0 gizzard shad in Oneida Lake. TheDaphniapopulation in Oneida Lake collapsed during July 1991, andDaphniaconsumption by gizzard shad dropped sharply with it; from dietary considerations, the collapse could have been caused by a density of approximately 300,000 age-0 gizzard shad per hectare. In 1992,Daphniaproduction remained strong during July and August, andDaphniaconsumption by gizzard shad did not decline. Because young gizzard shad can switch from zooplankton to phytoplankton as an alternative food source, we hypothesize that gizzard shad have a greater influence on growth and survival of age-0 yellow perchPerca flavescensthan yellow perch have on gizzard shad, because yellow perch rely more onDaphniaas a primary food.