Pairing of Dl, F2 and F9 isolates of Pythium sylvaticum with the known male (♂) and female (♀) isolates revealed that all isolates are males. No oogonia and oospores were observed when the individual isolates were cultured on corn meal agar, 10% V-8 agar contained 0.2% calcium carbonate, 2% water agar or potato dextrose agar plates. Matured oospores formed when the individual isolates of Dl, ♂ and ♀ were cultured in the different concentrations of V-8 juice contained 0.2% calcium carbonate, while no oospores were formed in V-8 juice without adding calcium carbonate. Using filter membrane to separate male and female isolates on corn meal agar revealed that all male isolates produced substances stimulating the oospore formation of female isolate. It is suggested that Dl is a weak self-fertile male, F2 and F9 are none self-fertile male,♂ is a strong self fertility and ♀ is a weak self fertility.