Within a single site in the Kohala Forest Reserve, Hawai‘i, we examined composition and diversity of soil microbial communities under four introduced (Cryptomeria japonica, Casuarina equisetifolia, Araucaria columnaris, and Eucalyptus sp.) and one native (Metrosideros polymorpha) canopy tree species, as well as pasture. Terminal restriction fragment length polymorphism (T-RFLP) analysis of soil bacteria, fungi, and archaea indicated that soil under the native M. polymorpha had the highest richness and greatest number of unique terminal restriction fragments, whereas soil under Eucalyptus and in pasture sites had the lowest richness. The soil microbial community differed significantly between Eucalyptus and M. polymorpha but not between the other three introduced species and M. polymorpha. The Eucalyptus microbial community was more similar to that of an adjacent deforested pasture site than to those of other forested stands. Soil pH was the environmental variable that best correlated with the variation in soil microbial community composition between tree species.