Multiple isozymes of glycerol-3-phosphate dehydrogenase (L-gIycerol-3-phosphate: NAD oxidoreductase, E. C. 1.1.1.8) have been resolved by starch gel electrophoresis of extracts of muscle tissue from lake whitefish (Coregonus clupeaformis). The isozyme electropherograms show that three kinds of subunit, A, B, and C, are synthesized in red muscle and two of these, A and B, are also found in white muscle. In red muscle the subunits evidently combine to form catalytically active dimers of the following types: AA, AB, BB, BC, and CC. In white muscle only the AA, AB, and BB dimers were observed.A genetic and molecular structure model is proposed for the glycerol-3-phosphate dehydrogenase (G-3-PDH) isozymes in white muscle of lake whitefish. On the basis of two alleles for A subunits and three alleles for B subunits, the model predicts a total of 18 distinct, electrophoretic G-3-PDH phenotypes for all possible AA, AB, and BB dimeric isozymes. The model has been confirmed by the results of a breeding experiment that tested the heritability of each of the five known alleles. A difference in the rate of heat inactivation of AA and BB isozymes was also interpreted as additional evidence for the unique genetic and molecular nature of the two kinds of subunits.Surveys of natural populations of lake whitefish revealed some marked variations in the frequencies of G-3-PDH alleles in fish from different geographical areas.The cisco (Coregonus artedii) also appears to have three "b" alleles for G-3-PDH.