Species hybrids were created via fusion of zoospores of two morphologically distinct species, P. infestans and P. nicotianae. Sixteen putative hybrid isolates were recovered that expressed differential drug resistance of each parent. Repetitive DNA of P. nicotiane was detected readily in all of these isolates by hybridization with the species-specific DNA probe, pPP33A. DNA of P. infestans was detected in only two putative hybrid isolates using PCR and primer pair ITS3 and PINF2. The two true hybrids were more similar to P. nicotianae than to P. infestans on the basis of pathogenic, morphological and molecular evidence. Additionally, hybrids expressed modified host ranges compared to parental species. Fusion of zoospores or hyphae may contribute to formation of such hybrids, particularly in the case of heterothallic species in which the joint occurrence of compatible mating types is rare. Zoospore fusion may prove useful as a tool to study hybridization, pathogenesis, and sources of natural diversity of species.