Differences in tissue patterns of enzyme-locus expression were used to infer the extent of divergence among species. Enzyme activities, at 27 loci in six tissues of 15 individuals in four species of Umbridae (Salmoniformes), were estimated on the basis of starch-gel electrophoresis of tissue extracts subjected to twofold serial dilutions and subsequent histochemical staining to determine the visual endpoints. Some locus-tissue expressions diverged markedly among species, while other locustissue expressions were conserved. Differences in activity for a given enzyme among tissues, as well as for different enzymes within the same tissue, were sufficiently independent to permit each locus-tissue expression of a species to be treated as a separate character. Statistically significant differences in levels of tissue-enzyme activities among species were then used to construct a phylogeny. The phylogeny constructed using tissue-enzyme expressional differences was similar to that based on enzyme structural differences (genetic distances) and to one of the morphologically based phylogenies. This congruence suggests that species differences in tissue patterns of enzyme-locus expression can be used to test a variety of systematic and evolutionary hypotheses.