Abstract About 16 fungal communities were isolated from the rhizospheres of thick (0.5–1 cm diameter) and thin (0.5–1 mm diameter) roots of living trees and stumps of common oak ( Quercus robur ). The density of fungi was 2–5× greater on thick roots from stumps than from living trees. The diversity of fungi was similar in the living trees and stumps. Some of the fungal species whose density was greater in rhizospheres of stumps than of living trees, e.g. Chrysosporium merdarium , C. pannorum , Cylindrocarpon destructans , C. didymum , Mortierella gracilis , M. hygrophila , M. microspora var. macrocystis , M. vinacea , Penicillium adametzii , P. daleae , P. janczewskii , Phialophora cyclaminis , Pseudogymnoascus roseus and Sporothrix schenckii , stimulated the formation of rhizomorphs of Armillaria ostoyae and A. gallica in oak-wood segments. It is presumed that the increase in density of fungi stimulating the rhizomorph production may favour the infection of oak stumps by A. ostoyae and A. gallica .