Treatment with the polyene antibiotic filipin resulted in the formation of granular protuberances of the plasmamembranes of the mesophyll cells of bean (Phaseolus vulgaris) and oat (Avena sativa) plants, and of intercellular hyphal cells of the rust fungiUromyces appendiculatus var.appendiculatus andPuccinia coronata var.avenae, as seen by freeze-etch electron microscopy. The granules were also occasionally seen in intracellular vesicles ofU. appendiculatus. None were found in any intracellular organelles of plant or fungal tissue. The granules ranged in size from about 20–25 nm in the plant tissue and 21–27 nm in the fungal tissue. They were concluded to be filipin-sterol (FS) complexes. The extrahaustorial membranes of either bean or oat rustinfected tissue were generally devoid of FS complexes. The extrahaustorial membrane is continuous with the host plasmamembrane but appeared to have a lower sterol content as compared to the non-invaginated plasmamembrane. The results are discussed in relation to membrane associations at the host-parasite interface.