Inoculated plants were generally ectomycorrhizal by 87 d, although in many cases 10 00 or less of their fine root length was colonized (Table 2). This level of colonization is comparable with that observed for the roots of uninoculated control plants and, in some cases, could therefore have been a contaminant in these treatments. There was large variation, both between fungal species and between isolates of the same species, in the ability of fungi to colonize plant roots (Harvest 2 data presented in Table 2). The most effective ectomycorrhizal fungi were generally those which colonized roots extensively, so that there were positive relationships (r2 = 0-79-084) between mycorrhizal colonization and plant growth (Fig. 2). However, for some fungi plant growth responses to inoculation could not be related to the length of colonized root. For example, some Cortinarius, Hysterangium and Amanita isolates increased plant growth yet did not appear to colonize roots extensively. Conversely, two Hydnangium isolates developed extensive mycorrhizas yet failed to increase plant growth.