Two forms of Salmo gairdneh with different life histories (steelhead and rainbow trout) were compared using restriction endonuclease analysis of mitochondrial DNA. A total of 19 individuals from four populations were studied for each of the two forms, using 14 restriction enzymes. In addition, five cutthroat trout samples were included as an interspecific comparison. These enzymes revealed a total of 81 cut sites, representing a sample of more than 400 nucleotides per fish. Of these sites, 25 were phylogenetically informative, dividing the 43 fish into 10 clonal lines, 8 Salmo gairdneri and 2 Salmo clarki. Results indicated detectable divergence between all geographic populations of steelhead and rainbow trout except Pennask rainbow trout, Coquihalla steelhead, and Wampus Creek rainbow trout. Other steelhead populations analysed showed a closer phylogenetic relationship to each other than to rainbow trout populations analysed. Intraspecific divergence was in most cases 1% or less, with a 1.5% maximum. Interspecific divergence between S. gairdneri and S. clarki was between 2% and 3.5%.