Nineteen isolates of Phytophthora infestans from potatoes growing in the Nile Delta were analyzed. All isolates were of the A1 mating type. Intraspecific isozyme variation was evaluated for all isolates. Using 15 selected enzymes, 215 electrophoretic phenotypes were identified among 19 isolates examined. Dihydrolipoamide dehydrogenase yielded the greatest number of electrophoretic phenotypes. Based on isozyme analysis, these isolates were subsequently divided into three distinct groups. Feulgen cytophotometry was used to measure the DNA content of individual zoospore nuclei. The distribution of DNA content in the Egyptian isolates was similar to that previously observed in isolates from other regions of the world.