Phytophthora insolita is a new species isolated from soil in Taiwan. The fungus produces oogonia but no discrete antheridia. Sporangia are noncaducous, ovoid, and without distinct apical papillae. Differentiation of zoospores precedes their release from sporangia. Hyphal swellings are variable in shape and size, and chlamydospores are thin walled and terminal. The fungus grew 8.4 and 4.0 mm/da at 35 and 38 C, respectively, and was not sensitive to 3-hydroxy-5-methylisoxazole. It was capable of inducing oospore formation of A2, but not A1 mating type of Phytophthora parasitica.