To explain diel predator-prey interactions between young red sea bream (pagrus major) and gammaridean amphipods, fish and macrobenthos were collected at 1-4h intervals daring a 24h period from June to July in 1981 and 1983. Young red sea bream showed two feeding peaks, a higher one at dusk and a lower one at dawn, and ceased to feed after dark. Gammarids were the most important food item for the young, but individual species were not taken in proportion to their abundances in the field. The patterns of gammarid selection by the young were related most closely to the microhabitat of gammarid species: Epifaunal and shallow burrowing specieswere positively selected; infaunal tube-dwelling species were positively or negatively selected with diel time; deep burrowing species were negatively selected. The intensity of predation on Byblis japonicus (infaunal tube-dwelier), the most dominant species in the field, was low about noon but increased remarkably at dusk and dawn, whereas that on Synchetidiunm miraculum lenorostralum (shallow borrower) and Paradexamine marlie (epifauna) increased about noon. This diet dietary shift is caused by diel vertical movements of gammarids, because the vertical movements change their microhabitat and consequently influence the availability of gammarid species.