Ectomycorrhiza formation by 11, geographically distinct Eucalyptus species and Pinus radiata with 23 diverse ectomycorrhizal fungi was examined in pure culture syntheses. Few differences occurred between the Eucalyptus species in their ability to form ectomycorrhizas with several fungi, thus indicating no evidence for host-fungus specificity within Eucalyptus. Pinus radiata and most Eucalyptus species formed ectomycorrhizas in common with several broad-host-ranging fungi. However, fungus species which are known to associate exclusively with members of the Pinaceae, e.g. Suillus and Rhizopogon species, did not form ectomycorrhizas with any Eucalyptus species and vice versa. Possible incompatibility between several host-fungus combinations was characterized by host accumulation of phenolic compounds in epidermal and cortical cells. These results are discussed relevant to better understanding degrees and processes of ectomycorrhiza host-fungus specificity and compatibility and to practical considerations in reforestation of exotic plantations.