Ecologically important ectomycorrhizal (EM) associations are poorly known from equatorial rain forests of South America. Recent field studies in the Pakaraima Mountains of western Guyana revealed previously undocumented forests dominated by EM leguminous trees, with a rich assemblage of EM mycobionts. Along transects, basidiomes from 75 species or morphospecies of putatively EM fungi were spatially associated with leguminous host trees. These fungi belonged to the basidiomycete families Boletaceae, Amanitaceae, Russulaceae, Cortinariaceae, Cantharellaceae, Clavulinaceae, and Entolomataceae, all of which are poorly documented from the lowland neotropics. Ectomycorrhizas were confirmed on D. corymbosa, D. altsonii, and D. jenmanii (Caesalpiniaceae, tribe Amherstieae), and a fourth species, Aldina insignis (Papilionaceae). The tribe Amherstieae contains most of the EM leguminous species forming monodominant forests in Guineo-Congolian Africa. Dicymbe species constituted the first record of EM Amherstieae in the New World. A variety of other co-occurring caesalpiniaceous trees failed to exhibit ectomycorrhizas. Transect surveys indicated that D. corymbosa and D. altsonii were: (1) highly clumped and dominant at specific sites; (2) occurred on soils with widely varying chemical and textural characteristics; and (3) the most important hosts for EM fungi in the local landscape. Dicymbe species have life history attributes, including the ectomycorrhizal habit, which enhance their competitive abilities irrespective of soil conditions. The spatial restriction of EM fungal basidiomes indicated that discrete groves of EM trees harbour an important component of regional macromycete diversity.