Because peak abundance of larval gizzard shad (Dorosoma cepedianum) occurs simultaneously with the mid-summer decline of macrozooplankton in Ohio reservoirs, I hypothesized that zooplanktivory by larval shad caused this decline. To test this hypothesis, I compared larval food consumption with zooplankton productivity in two reservoirs. Larval gizzard shad only partially controlled zooplankton production in a reservoir with high zooplankton productivity (exceeding 125 mg•m^-3•d-1), even at high larval densities (38 shad•m^-3). However, consumption by early juvenile (25-30 mm) gizzard shad severely reduced zooplankton in this reservoir. Conversely, larval shad exhibited nearly complete control of zooplankton in a reservoir with low zooplankton productivity (at most 4 mg•m^-3•d-1), even at relatively low larval densities (3-7 shad•m^-3). Depending on larval gizzard shad density and zooplankton production, larval shad alone or in conjunction with early juveniles can control zooplankton assemblages, increasing competition for limited resources at a time when zooplankton are critical to sport-fish recruitment.