The symbiosis of endotrophic fungi with Calluna, Erica and Vaccinium species was studied in artificially infected seedlings and cuttings. The fungi investigated are not species-specific, e.g., a fungus isolated from the European Vaccinium oxycoccus can serve the North American V. macrocarpon as a symbiont. When added to sterile cuttings the fungus stimulates root formation, and both with seedlings and cuttings it enhances growth. These symbiotic fungi have cellulase and pectinase. The process of penetration into the epidermis has been studied electron microscopically. The protoplast of an invaded epidermal cell is not killed by the fungus which eventually is digested by the living plant cell. The fungus cannot invade the subepidermal cells, probably because the secondary cell wall of the latter consists of, or contains, multiple layers of suberin. However, break-down products of the digested mycelium may not be prevented by the suberin layer from being assimilated, as radioactive serine was found to be taken up when applied ca. 1 cm from the tip of the root.