Genetic bases of isozyme phenotypes of alkaline phosphatase (AKP) and glucosephosphate isomerase (GpI) from tuber extracts of potato species of the genus Solanum were investigated by starch gel electrophoresis. Data were obtained from reciprocal F1 matings of S. tuberosum X ssp. andigena (Juz. & Buk.) Hawkes and ssp. tuberosum X (S. phureja X S. chacoense) and BC1 matings where ssp. tuberosum was the recurrent parent. AKP and GPI are dimeric enzymes and the variation observed for each was found to be coded by single tetrasomic loci (Akp and Gpi) with three (A, A', A″) and five (G, G', G″, G‴, G[Symbol: see text]) alleles, respectively. Although the G‴ and G[Symbol: see text] encoded homodimers have similar electrophoretic mobilities, the specific enzymatic activity of the G[Symbol: see text] encoded homodimer is approximately 25% that of the G‴. The predictable genetic bases for these two enzymatic polymorphisms make them suitable for use as genetic markers in the potato. Chromosome mapping of the loci which encode these enzymes is now possible.