Severe root rot of raspberry in Europe and North America is caused by a homothallic, non-papillate Phytophthora sp., which has been identified by different investigators as P. erythroseptica, P. fragariae, or as a highly pathogenic variant of P. megasperma. Two collections of such highly pathogenic raspberry isolates from Europe and North America were compared with recognized specimens of the 3 above named species, which were originally isolated from potato, loganberry and strawberry, and a variety of perennial host plants, respectively. All such raspberry isolates, regardless of previous identification, formed an essentially homogeneous group with respect to colony and growth characteristics, the production and morphology of gametangia, the morphology and dimensions of sporangia, electrophoretic banding patterns of mycelial proteins and pathogenicity. They were readily distinguishable from recognized isolates of P. erythoseptica and P. megasperma with respect to cultural, morphological, and electrophoretic criteria. In contrast, they were very similar to isolates of P. fragariae from strawberry (and a single isolate from loganberry), although the 2 groups could be separated by differences in growth rate on some agar media, production of oospores in culture, and small differences in electrophoretic banding patterns, as well as in pathogenicity. It is concluded that the raspberry isolates should be assigned to P. fragariae, but that they should be given a subspecific epithet at the varietal level to distinguish them from strawberry isolates of the species, i.e. P. fragariae var. rubi.