The influence of mycorrhizal associations on tree diversity patterns across tropical rain forests is poorly understood.  At the extremes of the tree diversity continuum, a general pattern exists:  low-diversity, monodominant trees are frequently ectomycorrhizal (ECM) and most other trees in high-diversity rain forest are arbuscular mycorrhizal (AM). One hypothesis to explain this pattern is that ECM fungi may suppress saprotrophs and ‘short-circuit’ organically-bound nutrients back to their host tree.  Unlike AM fungi, ECM fungi are derived from saprotrophic lineages and retain the enzymatic capacity to mobilize organic sources of limiting nutrients. However, despite these capabilities, ECM fungi are poorer competitors for organically bound nutrients than saprotrophs, and there is evidence that suppression of saprotrophic competitors may occur to compensate for ECM fungi's poorer saprotrophic abilities. To test this hypothesis, we studied a monodominant forest in Guyana, where the ECM tree Dicymbe corymbosa forms >80 % of the canopy, and a high- diversity, mixed forest where most trees are AM.  Previous field data show that decomposition and litter turnover in the monodominant forest is significantly slower.  To determine if ECM suppression of saprtrophs could account for these results, we used phospholipid fatty acid (PLFA) analysis and denaturing gradient gel electrophoresis (DGGE).  We found that microbial biomass is higher in the mixed forest and the dominant fungal groups in the mixed forest are saprotrophic, whereas ECM fungi dominate in the monodominant forest.  We then constructed clone libraries from both forests and determined that saprotrophic diversity is significantly higher in the mixed forest.  Together, these results suggest that ECM fungi may be suppressing saprotrophic fungi in the monodominant forest, and may be one mechanism by which ECM trees become locally dominant in tropical rain forests.