Fish materials for the investigation of stomach contents were obtained from catches by the coastal trawl fishery in Sendai Bay. The various food organisms for the demersal fishes were classified into three groups in accordance to the differences in the degree of dependance to the bottom, and these are pelagic fauna, epifauna and infauna. Judging from the composition of these three life forms of the food organisms, demersal fishes can be classified clearly into three feeding types, namely pelagic fauna feeder, epifauna feeder and infauna feeder.
The operation records of the coastal trawl fishery were used for investigating coaction concerning habitat selection among the fishes belonging to the same feeding type. The habitat segregate among the fishes belonging to the infauna feeder, showed the existence of coaction in the selection of habitat. The coaction becomes more severe with increments in the degree of resemblance of feeding habit between two species.
The overall density of the infauna feeder increased with the decrease of grain size of bottom sediment. Entirely different from the case of the infauna feeder, the most densely living place for the entire epifauna feeders, is in the relatively hard bottom area of 2-3φ of mean diameter of bottom sediment. These relationships show the same tendency with the relationships between the density of epifauna or infauna and the grain size of bottom sediments in Long Island Sound and Buzzard Bay. Among fishes belonging to the same feeding type in the region of same mean diameter of grain size range, MOTOMURA's law of geometrical progression can be applied. From these facts, it is thought that the entire density of the fishes belonging to each feeding type is restricted by the density of infauna and epifauna, respectively.

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