Diel changes in the distribution of the tidewater silverside (Menidia beryllina) and its prey were measured in Clear Lake, California, a large, shallow, eutrophic lake. Zooplankton prey densities were low in the littoral zone, but increased rapidly to near peak abundances 50–200 m from shore. Gill netting, trawling, and visual observations showed that tidewater silversides migrated both horizontally and vertically to feed in areas of high zooplankton abundance. At night the fish concentrated near shore and did not feed. At dawn (8 × 108−2 × 1010 photons cm−2 nm−1 s−1) the fish migrated lakeward at least 50–1,000 m, and littoral abundance decreased from more than 100 m−2 to only 0.1 m−2. The migration preceded the initiation of feeding by 30–60 min. After feeding 2–4 h, the fish returned to the littoral zone and swam rapidly (6–11 body lengths s−1) parallel to shore in narrow, continuous schools until resting aggregations formed. The fish returned to shore before they were satiated, perhaps to balance predation losses in offshore areas against foraging gains. A second period of offshore activity and feeding occurred in the afternoon and evening.