Mitochondrial (mt) DNA restriction endonuclease assays of seven populations of Oncorhynchus tshawytscha from British Columbia and Alaska disclosed six distinct maternal (clonal) lines, of which three were represented by more than one individual. The three major lines were not population specific. Of the British Columbia fish sampled, 41 belong to clonal line A (82% of the 50 fish sampled in British Columbia). Clonal line C contained 10 fish from Alaskan sites only. Clonal line B contained 20 fish from both British Columbia and Alaska sites. Intraspecific mtDNA sequence divergence is low (less than 1% in ail cases) compared with other species surveys of this sort. Comparison with protein electrophoretic studies of O. tshawytscha reveals similar levels of population structuring. A database of mtDNA fragment sizes produced by the 14 restriction endonucleases is included.