Density and spatial distributions on an intermediate scale of two flatfishes—marbled flounder Pleuronectes yokohamae, and the brown sole Pleuronectes herzensteini (Pisces: Pleuronectiformes) within large (10,000–100,000 m2 patches of homogenous substratum (sand and silted sand)) were studied. Both species dominated in the demersal fish community. Densities of brown sole, were significantly higher in both types of habitat. Flatfish distribution was highly patchy and the mean fish densities in groups were as much as ten times higher than the mean fish densities in the area. More than 85% of all recorded flatfish were found within groups. Mixed groups with both species were most prevalent. The number of flatfish in one group ranged from 2 to 20. Brown sole was more abundant in the area and dominated in mixed groups. Levels of contacts between group members and densities within groups were inversely related to visibility condition. It is suggested that flatfish form a group and stay in the group, attracted by foraging movements of other flatfish and by plumes of upset particles caused by their bottom digging movements.