The haustorial apparatus of A. candida consisted of a narrow neck attached to a spherical body, which was packed with mitochondria. Covering the haustorial body was an electron-dense matrix which was bounded by the extrahaustorial membrane. This membrane, a continuation of the host [here, Brassica oleracea var. capitata cv. Wheeler''s Imperial] cell plasmalemma, was highly convoluted with many short tentacles extending into the surrounding host cytoplasm. Single cisternae of host rough endoplasmic reticulum were always closely associated with the distal portion of the haustorial neck. Silver proteinate staining indicated that both the penetration jacket, a distinct zone surrounding the proximal part of the haustorial neck, and the extrahaustorial matrix were polysaccharide in nature, possibly complexed to other macromolecules such as proteins. The haustorial cell wall reacted only slightly with the silver proteinate, and in the distal region of the neck, there was no reaction. The fungal plasmalemma reacted strongly with the stain, except in the distal neck region, where there was no staining. With PACP [periodic acid-chromic acid-phosphotungstic acid], a staining procedure believed specific for the plasmalemma, the extrahaustorial membrane did not stain, while the host plasmalemma continuous with it did stain. The distal region of the haustorial neck wall also stained intensely with PACP clearly differentiating it cytochemically from the much lighter staining proximal neck and haustorial body walls. The penetration jacket stained more intensely than the extrahaustorial matrix with PACP, which might suggest a different chemical composition. Following incubation in 0.8 M sucrose osmoticum, the extrahaustorial membrane pulled away from the proximal neck wall, but remained tightly affixed to the distal neck wall, indicating that a tight junction existed between that membrane and the fungal cell wall in that region. Enzymic digestion with either cellulase or an endo-.beta.-1,3-glucanase removed neither the haustorial body wall nor the extrahaustorial matrix. The significance of this finding is briefly discussed.