Diets of flatfishes and other demersal fishes and relations between their assemblages and macrobenthos were investigated by examining stomach contents of 32 species of demersal fishes, and seasonal abundance of macrobenthos collected off the coast of Tomakomai. Flatfishes Hippoglossoides pinetorum, Pleuronectes herzensteini and Kareius bicoloratus fed exclusively on polychaete Onuphis holobranchiata through the year. Diet of other main demersal fishes Spirinchus lanceolatus, Eleginus grcilis, Myoxocephalus jaok, Podothecus sachi, Arctoscopus japonicus, varied in each season. They fed chiefly on juvenile fishes in June and August, Onuphis holobranchiata in August and September, Mysidacea and Macrura in December. There was considerable influence of seasonal abundance and size utilization of prey species in food selection of predators. Especially Onuphis holobranchiata recruited in summer and became an important food resource for many fishes.