Relatedness of Group I species P. iranica, P. clandestina, P. pseudotsugae, P. cactorum and a new species, P. idaei with specificity in pathogenicity tests to raspberry, were examined at the molecular level using 20 random ten-mer primers to amplify total DNA (RAPDs) and by sequencing of the internal transcribed spacer regions, ITS1 and ITS2, of the ribosomal RNA gene. Cluster analysis of 5 of the RAPD banding patterns, separately or as a combined analysis, ranked P. idaei, P. pseudotsugae and P. cactorum more or less equally in each case. The separation between them and P. iranica and P. clandestina was much greater. Within P. cactorum collar rot isolates from apple clustered separately from strawberry crown rot isolates, while isolates from raspberry appeared to have affinities with both clusters. Sequence analyses of ITS1 and ITS2 revealed only a few differences among isolates of P. idaei, P. pseudotsugae and P. cactorum. Their sequences were much more similar to one another than to P. infestans and in ITS1 they shared a characteristic motif, resulting from a base pair inversion, that was not present in any other Phytophthora species. The inversion occurred within a sequence that binds to a DNA probe that has been used as a genus-specific probe for all Phytophthora spp.