Isozyme polymorphisms among 59 isolates of Mucor piriformis from pears and nectarines were examined by vertical polyacrylamide gel electropho- resis. Among the enzyme systems tested, six enzyme activities, catalase, cx-esterase, glucose-6-phosphate de- hydrogenase, lactate dehydrogenase, malate dehy- drogenase and superoxide dismutase were used for detailed analyses. The variability of isozyme patterns revealed seven electrophoretic phenotypes for all the enzymes. No distinguishing morphological features were associated with the different electrophoretic phenotypes. In contrast, there was a correlation be- tween esterase, glucose-6-phosphate dehydrogenase and malate dehydrogenase banding patterns and the mating potency of the isolates from pears; those with mating abilities (plus and minus isolates) were clearly different from those without mating abilities (neutral isolates). In fruit inoculation pathogenicity studies, the plus and minus isolates were equally pathogenic to the neutral isolates. The 30 isolates from nectar- ines proved to be very homogeneous genetically, they did not reveal any polymorphisms for the 6 enzymes investigated. The potential application of the isozyme analysis to provide biochemical markers for Mucor species is discussed.