The white-rot basidiomycete, Inonotus hispidus, produces a characteristic pattern of decay in naturally infected London plane (Platanus × hispanica) in which zones of severely decayed wood, traversed by relatively undegraded xylem rays, alternate with relatively undegraded, gummous ‘reaction zones’. Microscopic observations of London plane wood both naturally colonized and artificially inoculated with I. hispidus indicated that the resistance of reaction zones and xylem rays to degradation is due respectively to obstruction of cell lumina by tyloses and polyphenols, and infiltration of intercellular spaces by polyphenols. Inonotus hispidus was apparently able to circumvent these impedances by forming soft-rot like tunnels through the wood cell walls.