The morphology, physiology, protein patterns and pathogenicity of a papillate Phytophthora sp. that is widely distributed in raspberry stocks, are described and compared with those of other Group I Phytophthora species, especially P. cactorum, and with species from other groups that attack raspberry (P. citricola and P. syringae). The papillate species, although similar to typical isolates of P. cactorum from raspberry and other hosts, could be distinguished from them on virtually all the criteria examined. Distinctive characteristics that separated them from P. cactorum isolates were absence of growth patterns on certain media; persistent sporangia; low rates of growth; low cardinal temp.; inability to utilize nitrate nitrogen; host range limited to raspberry but with more severe symptoms; and differences in protein patterns. It was concluded that these papillate Phytophthora isolates from raspberry represent a new species which has been named P. idaei sp.nov.