Abstract: Pyrimorph is a novel fungicide developed by China Agricultural University and Institute of Plant Protection.Bioassay of pyrimorph to Phytophthora infestans was conducted.The results showed that the toxicity of pyrimorph was high in controlling P.infestans.It could intensively inhibit the mycelium growth,sporangium production and cystospore germination of P.infestans and the EC50 value was 0.066, 0.059 and 0.550 μg/mL, respectively.It was insufficient in inhibiting the release of zoospores from sporangia and the EC50 value was 9.78 μg/mL.Microscopic observation of pyrimorph treatment showed that the mycelium branch was fewer and the space was longer than that of CK.The difference of hypha diameter was not distinct between pyrimorph treatment and CK.When P.infestans invaded inside tomato leaves,it showed that only a few cystospores germinated 24 hours after inoculation and there was no sporangium 96 hours after inoculation of pyrimorph treatment at 100 μg/mL.Pyrimorph was found to influence on the permeability of the cell membrane of P.infestans at 50 μg/mL,and the difference was not distinct at 1 μg/mL and 10 μg/mL.It showed that pyrimorph had some effect on the protein biosynthesis of P.infestans at 100μg/mL,but had little effect on the DNA biosynthesis.