Abstract: Besed on classical assay of mating type of heterothallic pathogen, total 186 isolates of Phytopathora infectans from representative potato and tomato planting areas in 5 counties and 3 districts in Chongqing have been determined on RSB and RTA paltes using known mating type. The mating type assay showed that no relative A1 mating type of P. infectans with oospore was found in Chongqing.Namely, the whole isolates tested were all A1 mating type.