Stomach contents of juvenile Japanese Bounders were studied from 3 sheltered and 3 exposed bays. Newly settled Bounders (15-20 mm in TL) mainly consumed mysids regardless of the availability of food organisms. In sheltered bay areas where mysid densities were low, Bounders larger than 60 mm preyed mainly on juvenile fish. In exposed inshore areas mysids were abundant and constituted the major dietary item of flounder until 200 mm. 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 Bounders (from 15 mm to 600 mm) and the size and number of prey consumed suggest that small mysid individuals are the most suitable prey items for flounders until 50 mm and fish prey items are essential for larger flounders.