Gut contents of five dominant copepods, Acartia hudsonica, A. sinjiensis, Eurytemora paclfica, Sinocalanus tenellus and Oithona davisae, occurring in the Lake Naka-umi from January to September 1997 were examined with scanning electron microscopy. All the species fed on dominant phytoplankters such as Prorocentrum minimum, Cyclotella spp., Thalassiosira spp., Skeletonema costatum, and Minidiscus comicus, and rarely on benthic pennate diatoms, suggesting that these species are opportunistic feeders and show little food segregation within the copepod community. In April 1997 a dinoflagellate Noctiluca scintillans was blooming, and voraciously fed on P. minimum, copepod eggs and zooplankton fecal pellets, which may more or less influence the occurrence of a red tide phytoplankter P. minimum, population growth of pelagic copepods, and flux of sinking particles. The feeding ecology of other dominant zooplankters in Lake Naka-umi, such as appendicularians, chaetognaths, cladocerans, mysids, rotifers and veliger larvae, were reviewed.