When eucalypt mycorrhizal fungi were separated from eucalypt roots by interposing membrane niters, the fungal hyphae grew directly through these to contact the host root apices. The same mycelia grown on membrane filters over various agar media always remained above and readily detachable from the membranes, with no evidence of penetration. More significantly, these same eucalypt fungi did not grow through membranes towards the roots of several non-host plants under the same circumstances, and mycorrhizal fungi of other plants such as birch and pine failed to penetrate membranes overlying eucalypt roots. It seems likely that there is a selective chemotropic attraction of these mycorrhizal fungi to substances diffusing from compatible host root apices. Such chemotropism could provide the signal which initiates the ectomycorrhizal infection process in the root cap region, and guides the inward growth of the hyphae through that tissue during the early stages of sheath formation.