The behavior of chum salmon Onchorhynchus keta was studied using ultrasonic telemetry in the waters off the Okhotsk coast of Hokkaido from 1979 to 1981. Certain peculiar escape reactions wereobsered when chum salmon encountered Dall's porpoises. The salmon either dived quickly to the bottom in an apparent move to escape from the porpoise, or continued to swim around at the surface without showing any frantic attempt to swim away from the propoise. When they encountered the porpoise over deep waters (deeper than 100 meters), the salmon usually remained at the surface, but those that came across the porpoise while in shallower waters tended to dive quickly, and steeply, to the bottom.