Comparison of myxosporidian species compositions in different flatfish species fro Peter the Great Bay showed that the flatfishes should be divided into three ecological groups--deep water (scaly-eyed plaice Acanthopsetta nadeshnyi, Korean flounder Glyptocephalus stelleri, flatheaded flounder Hippoglossoides dubius, and pinewood flounder Cleisthenes herzensteini), relatively coastal (yellow-finned sole Limanda aspera and brown sole Pseudopleuronectes herzensteini), and coastal (Japanese flounder Pseudopleuronectes yokohamae, long-snouted flounder Limanda punctatissimus, starry flounder Platichthys stellatus, and white-bellied flounder Lepidopsetta mochigarei). This division coincides in general with the previously proposed ecological classification based on the characteristics of bathymetric distribution and preference to water layers during summer period in the fishes.