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Two isolates of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker and Couch were inoculated on to roots of Eucalyptus urophylla S. T. Blake in vitro and a comparison made of the rate of ectomycorrhizal development. Both isolates initiated ectomycorrhizal roots hut the rime of onset of root colonization and the rate of formation varied with the isolate An isolate cultured from a sporocarp collected from under eucalypts in Australia rapidly colonized roots of E. urophylla and initiated mycorrhizas after 2 d incubation, whereas an isolate obtained from sporocarps collected from under pines in the United States developed ectomycorrhizas only after 7 d incubation. Dual inoculation of E. urophylla seedlings using a paper-sandwich technique again showed the rapid colonization of roots by the eucalypt isolate even when roots were inoculated 7 d previously with the pine isolate. These in vitro observations support field observations that there are variable degrees of plant species - fungal isolate compatibility, implying specificity, and this is an important factor influencing successful ectomycorrhiza formation and development. [ABSTRACT FROM AUTHOR]