The reaction characteristics of histocytology in interaction between potato with horizontal resistance and Phytophthora infestans were examined using general microscopy and electron microscopy.Microexamination results showed that:(1) Hypersensitive response (HR) was observed in horizontal-resistance potato after inoculation and hyphae were restricted within the cells infected.Fewer branches of hyphae and haustoria were formed.(2) The spreading trend of invaded cells,more haustoria and hyphae branchses were found in susceptible cultivar.(3) The electron microscopy observation showed that a series of abnormal changes occurred in intercellular hyphae,haustorial mother cells and haustoria in the resistant potato cultivar.The cytoplasm became more electron-dense and numbers and sizes of vacuoles increased.The cell walls of hyphae and haustorial mother cells were thickened irregularly.The organelles were distributed disorderly in the cytoplasm and the shapes of haustorial mother cells and haustoria became abnormal.Finally,the intercellular hyphae,haustorial mother cells and haustoria became necrotic and collapsed.The structural defense reactions such as formation of cell wall deposition which made cell wall thickened extremely or papillar structures in the inner site of host cell wall were also observed in the resistant potato.