Abstract:Ultrastructural changes in leaves from a resistant and a susceptible Chinese cabbage (Brassica campestris ssp. pekinensis (Lour) Olsson )cultivar inoculated with Peronospora parasitica (Pers.) Fr. were studied with transmission electron microscopy. After the entry of the pathogen, the infected cells of resistant cultivar showed hypersensitive death. Necrotic cells with fibrillar material were observed. This Suggested that the resistance to P. parasitica was mainly expressed by the rapid necrosis of infected cells. Extensive phagosomes and plasmalemmasomes, swollen endoplasmic reticula, elongated mitochondria, increase in the number of ribosomes and Golgi vesicles were observed in uninfected cells of the resistant cultivar inoculated with P. parasitica. These ultrastructural changes appeared to be closely related to the resistance response to the incompatible pathogen . In contrast, convoluted membrance rings and a number of crystal-containing microbodies demonstrated in susceptible cultivar in the early stage of infection, also seem to be closely related to disease resistance.