Testes of mature chum salmon (Oncorhynchus keta Walbaum) were examined histologically for interstitial cells and extracts of testes assayed biologically for the presence of androgens. The histological examination showed numerous interstitial cells similar to those described in some other fish and in mammals. The bioassay established the presence of androgenic substance (probably testosterone). The amount present in salmon testes was calculated to be roughly equivalent to the amount present in mammalian testes.