Electrophoretic variation of sorbitol dehydrogenase has been found in Mus musculus; the SDH-1 A phenotype migrates further toward the cathode under the conditions used than the SDH-1B phenotype. Mice with the SDH-1B phenotype have enzyme activity reduced to 25% of that found in mice with the SDH-1A phenotype, but no differences in heat stability, pH profile, or Km for fructose were found. The gene Sdh-1 is inherited as an autosomal condominant located on chromosome 2, and the gene order obtained, with genetic distance calculated as percentage recombination +/- SE was Sdh-1--1.9 +/- 1.4--pa--16.4 +/- 3.6--a--3.9 +/- 1.9--bp.