During 349 hours of sighting surveys conducted from late June to early August 1979 in the northwestern North Pacific Ocean and the southern Bering Sea, 167 schools of Phocoenoides dalli were sighted. Three kinds of schools (dalli-type, truei-type and mixed schools composed of both types) were observed in the southern areas surveyed while only dalli-type schools were seen in the northern areas. School size of dalli-type schools was smaller than that of the truei-type or mixed schools. Truei-type as well as mixed schools tended to occur in warmer waters (above 13°C) than dalli-type schools during the second part of the survey. Behavior of animals in mixed schools was not apparently different than in the segregated schools. Mixed schools were composed of all sizes of animals including an adult-calf pair. The relationship between the two color types is discussed.