Specific activities and electrophoretic mobilities of glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase were determined in 38 isolates of the family Enterobacteriaceae and in 10 isolates of the related Pasteurella. The deficiency of glucose-6-phosphate dehydrogenase in P. pestis was verified. Enzymes obtained from different strains of the same species exhibited an unexpected degree of heterogeneity. For example, 8 and 11 apparent variants of glucose-6-phosphate dehydrogenase and phosphogluconate dehydrogenase, respectively, were found in 14 strains of Escherichia coli. Although similar frequencies of heterogeneity were noted in 7 strains of P. pseudotuberculosis, 5 species of Shigella, and 8 species of Salmonella, differences in mobility were generally small in comparison with those observed between strains of E. coli. Values obtained for the pasteurellae, shigellae, and salmonellae, thus fell within narrow ranges that may prove typical for the genera. However, most of these ranges, as well as many values observed for single species of other genera, were overlapped by the wide range recorded for E. coli. The significance of this observation was discussed with respect to the relative age and taxonomic position of the organisms in question. The method could be used to distinguish between most wild-type strains of the same species and should thus facilitate investigations of genetic transfer and epidemiology.