Previously reported results on natural populations of brown trout (Salmo trutta L.) urged further studies using additional electrophoretically detectable loci. Samples of brain, eye, heart, kidney, liver, and muscle from approximately 50 specimens from each of two populations and a large number of muscle samples from several additional populations were examined. Electrophoretic and staining methods for the 37 enzymes studied are given in detail. Of a total of 69 loci detected, 54 loci were considered usable in population genetics screenings. The expression of the loci coding for these enzymes is described and interpreted. The results presented will serve as a basis for a more detailed examination of genetic variation in brown trout populations.