Data from family lots of chinook salmon Oncorhynchus tshawytscha whose parents had variable electrophoretic phenotypes for adenosine deaminase (ADA) indicated (1) that each of two distinct zones of ADA activity reflected a single genetic locus, (2) that allelic variants that segregated in a Mendelian manner occurred at both loci, and (3) as a consequence of storage, additional isozymes arose from products of the locus encoding the more cathodal zone (ADA-1). The additional bands complicated genetic interpretations, and their occurrence was reduced by treatment of tissue samples with 2-mercaptoethanol. These data permit the use of ADA isozymes in population studies of chinook salmon, and provide a model for interpreting ADA variation in other salmonids.