Aseptic oospores, presumed to be those of P. fragariae, were obtained by a blending and sieving technique from strawberry roots showing symptoms of red core. Some usually germinated within 7 days of being placed on distilled water agar. Incubation for 24 or 48 h at 25 deg C before plating on the agar greatly increased the proportion of oospores which germinated but exposure to 35 deg C for the same times was deleterious. Germinating oospores either gave rise to one or more sporangia or formed a branched mycelium. Cultures grown from hyphal tip isolates prepared from germinated oospores were typical of P. fragariae, and their identity was further confirmed when strawberry plants inoculated with them developed red core symptoms.