As part of a continuing study of non-host resistance, red clover leaves inoculated with the clover pathogen Stemphylium sarcinaeforme, or the closely related alfalfa pathogen S. botryosum, were examined by light and electron microscopy to compare the events occurring in the initial stages of infection. Stemphylium botryosum penetrated leaves primarily via the stomata with resultant death of the guard cells and with varying effects on adjacent epidermal cells. Appressoria were frequently formed, and although they rarely resulted in successful penetrations, the contacted epidermal cells were often markedly affected as judged by toluidine blue staining. Growth of hyphae was intercellular but very limited in its extent. At some infection sites, one to several mesophyll cells underwent extensive plasmolysis and cytoplasmic disruption. Less severely affected mesophyll cells contained large lipid bodies, abundant rough endoplasmic reticulum, and Golgi vesicles and had wall appositions at points of contact with necrotic cells or with hyphae. Stemphylium sarcinaeforme generally penetrated between or directly through the epidermal cells, causing death of the contacted cells. Hyphae grew intercellularly or intracellularly in the palisade tissue and hyphal elongation was considerably more rapid than that of S. botryosum. Palisade cells adjacent to, or containing, the hyphae underwent several different types of cytoplasmic deterioration. Mesophyll cells surrounding these areas showed the same features as comparable cells in tissue infected by S. botryosum. In tissue infected by either fungus, the exterior of host cell walls was coated with a layer of extracellular material.