Stomach contents of juvenile Japanese flounders were studied from 3 sheltered and 3 exposed bays. Newly settled flounders (15-20mm in TL) mainly consumed mysids regardless of the availability of food organisms. In sheltered bay areas where mysid densities were low, flounders larger than 60mm preyed mainly on juvenile fish. In exposed inshore areas mysids were abundant and con-stituted the major dietary item of flounder until 200mm. The amount of the stomach contents was not significantly different between the two area types. From monthly investigations it was found that seasonal changes in the feeding activity of juvenile flounders is closely associated with water temperature and mysid abundance. Analysis of the relationship between the total length of flounders (from 15mm to 600mm) and the size and number of prey consumed suggest that small mysid individuals are the most suitable prey items for flounders until 50mm and fish prey items are essential for larger flounders.
