Genotypes of Rubus spp. and some of their hybrids, grown in vitro, in a plastic tunnel and in the field were inoculated with P. rubi or exposed to inoculum of this pathogen to study histologically the infection of leaves, flowers, developing fruits and stems. Samples were fixed, stained and mounted in aniline blue and examined by fluorescence or differential interference contrast microscopy. Conidia germinated on adaxial and abaxial leaf surfaces and penetrated the epidermal cell walls to enter the mesophyll. Points of penetration were stained intensely, probably due to callose formation by the host. Some germ-tubes infected leaves by entering the stoma, but this was less common than direct penetration. Intercellular hyphae colonized the mesophyll extensively and formed simple dichotomously branched haustoria. Spread of the pathogen into the vascular tissues of veins was restricted and accounted for the interveinal distribution of lesions. Hyphae which entered the veins and petioles and spread to the cortical tissues of the stem tended to produce more complex haustoria, but less frequently than in other tissues. In petioles and veins of leaves, fanshaped laterally fused hyphal branches (fasciated hyphae) formed in intercellular spaces and evidence was obtained that anastomosis may have occurred between hyphae growing parallel within these tissues. Oogonia with paragynous antheridia and thick-walled aplerotic oospores developed abundantly in leaves grown in vitro. Oogonia also developed in petals and stamens from flowers of Tummelberry grown in the field and spray-inoculated in the lab. An extensive network of mycelium developed in the mesocarp and the receptacle of developing fruits.