The contribution of mycorrhizal associations to maintaining tree diversity patterns in tropical rain forests is poorly known. Many tropical monodominant trees form ectomycorrhizal (EM) associations and there is evidence that the EM mutualism contributes to the maintenance of monodominance. It is assumed that most other tropical tree species form arbuscular mycorrhizal (AM) associations and while many mycorrhizal surveys have been done the mycorrhizal status of numerous tropical tree taxa remains undocumented. In this study we tested the assumption that most tropical trees form AM associations by sampling root vouchers from tree and liana species in monodominant Dicymbe corymbosa forest and an adjacent mixed rain forest in Guyana. Roots were assessed for the presence/ absence of AM and EM structures. Of the 142 species of trees and lianas surveyed three tree species (the mono-dominant D. corymbosa the grove-forming D. altsonii and the non-dominant Aldina insignis) were EM 137 were exclusively AM and two were non-mycorrhizal. Both EM and AM structures wer e observed in D. corymbosa and D. altsonii. These results provide empirical data supporting the assumption that most tropical trees form AM associations for this region in the Guiana Shield and provide the first report of dual EM/AM colonization in Dicymbe species. Dual colonization of the Dicymbe species should be further explored to determine if this ability contributes to the establishment and maintenance of site dominance.