The first fertile hybrid generation ever known to have been produced artificially between salmon (Salmo salar L.) and trout (Salmo trutta L.) has been examined electrophoretically. To find out the mode of inheritance for the protein systems investigated the F1hybrids were compared with samples of salmon and trout of the parental populations as well as a sample of F2hybrids. All fishes were 2+ years of age. In the protein systems where protein or enzyme activity was detected 17 were more or less complete summations of the parental patterns in the F1hybrid thereby displaying a codominant relationship between the parental genes. Four protein systems were identical in both species. Dominance of trout genes was the most striking feature of the F2hybrids, since the action of all salmon genes was suppressed. In four patterns one or more zones had been added to the theoretical maximum pattern obtained by simply mixing the parental patterns. This phenomenon was attributed to the action of multiple gene systems displaying transgressive segregation.