Monthly changes in the main food items of and biased feeding on adult males of harpacticoid copepods by marbled sole Pleuronectes yokohamae juveniles were examined in the Coastal Area of Tsugaru Strait, Hokkaido, from April to September 1996 and in May 1997. At Iwabe Fishing Port in May-July 1996 and May 1997, marbled sole juveniles (≤30 mm in SL) fed on harpacticoid copepods, mainly Harpacticus sp. Off Nanaehama Beach in May 1997, juveniles preyed on harpacticoids, mainly Halectinosoma sp. In August, the main food organisms of juveniles (>30 mm in SL) changed from harpacticoids to small benthic crustaceans and polychaetes. In September 1996, juveniles fed chiefly on polychaetes. Adult males of Harpacticus sp. and Halectinosoma sp. were found in significantly larger numbers than adult females and copepodites (stage I-V) in fish stomachs. In contrast, no significant difference was found in the sex ratio of Harpacticus sp. on the bottom sediment at Iwabe. Because adult female harpacticoids in the stomachs of juveniles had larger body widths than adult males and copepodites, the biased feeding on adult males can not be explained by differences in prey size. The specific feeding by marbled sole juveniles might have been caused by a behavioral difference between harpacticoid males and females in their breeding season.