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AbstractAbstractAn efficient method by which to bring tropical leaf-litter fungi into culture was sought in order to survey these organisms for pharmacologically useful metabolites. A simplified particle-filtration procedure, based on the method of soil washing, was tested to determine if the characteristic fungal genera of leaf litter could be preferentially isolated while minimizing recovery of soil and common saprobic fungi. At the same time, we made some preliminary measurements of the magnitude of fungal species richness in tropical forest litter. Litter from four sites in a primary rain forest of the Osa Peninsula of Costa Rica were examined. Pulverized leaf litter from each site was separated into fine particles and repeatedly washed. From each leaf litter sample, a 1-ml particle suspension was plated onto two selective media. One-ml suspensions of leaf litter from the four samples yielded a total 1709 isolates, ranging from 281 to 559 isolates/sample (mean = 424). The number of species/sample ranged from 78 to 134. Rarefaction curves based on the number of species expected in random subsamples were used to compare species richness among samples. Many uncommon genera of litter fungi were recovered as well as coelomycetes, sterile strains, endophytes, and phy- topathogens. Typical soil fungi were a relatively minor component of the total isolates. Species-abundance distributions showed that there were few abundant species and a high proportion of rare species. In no sample did the single most abundant species account for more than 23% of the total isolates. Species present in all samples were Cylindrosympodiella sp., Glomerella cingulata, Lasiodiplodia theobromae, Pestalotiopsis guepinii and an unidentified coelomycete. Particle filtration of leaf litter appears to be practical for use in a microbial screening program because it circumvents the tedious process of hand isolation of uncommon genera of litter fungi. However, if endophytic or soil fungal communities from the same region are being examined simultaneously as reservoirs for screening isolates, then redundant taxa distributed vertically through different forest strata will need to be eliminated.Key Words: decompositionmicrobial diversityOsa Peninsulararefactionselective media