Leaf isozymes of 145 cultivars of grapes (Vitis vinifera L. and Vitis spp.) were separated via starch gel electrophoresis. They could be separated into 52 groups using glucosephosphate isomerase (GPI) and phosphoglucomutase (PGM) isozymes. Twenty-four cultivars had unique combinations of isozyme patterns. Two loci, Gpi-2 and Pgm-2, were identified and shown to be simply inherited with multiple alleles at each locus. The mean heterozygosity for the two loci (Gpi-2 and Pgm-2) among all the cultivars was 57.5%, indicating a high level of genetic variability among these cultivars. Indirect evidence for expressed linked lethal alleles was demonstrated by the missing classes in Gpi-2 and Pgm-2 segregation ratios (1:2:0 ratios) in several selfed progeny families. Chemical name used: methyl[1-(methylethyl)-1H-benzimidazol-2-y]-carbamate (benomyl).