This study focuses on the characteristics displayed by mesophilous and thermophilous microfungal populations occurring in two tropical monodominant plant communities, a Cocos nucifera grove and a Casuarina equisetifolia forest, that provide distinctly different edaphic conditions. The mesophilous population sampled at 25°C by the dilution plate method and the thermophilous population that developed on soil plates incubated at 45°C consisted of 1693 isolates representing 60 species and 29 genera and 8887 isolates representing 20 species and 10 genera, respectively. The mesophilous propagules averaged 9,990 per gram dry soil in the coconut grove that lacks a litter layer, is low in moisture and organic matter and is subjected to high solar irradiation. The population was characterized by the prevalence of aspergilli and dematiaceous-sphaeropsidaceous forms and the near absence of mucoraceous isolates. Ascomycetes were common. The only widespread taxa were the three species, Aspergillus niger, Penicillium chrysogenum, and Cladosporium cladosporioides. Species diversity was high and 73% of the isolates were cellulolytic. In the casuarina forest, adequate moisture and organic matter and a protecting litter layer provide a mesic environment. The mean number of mesophilous fungi per gram dry soil was 32,800. This figure is considerably lower than ones reported for mesic temperate communities and may be due to more rapid propagule removal through accelerated microfaunal and microbial activity. An abundance of mucoraceous and moniliaceous isolates and penicillia, and the rarity of aspergilli, dematiaceous-sphaeropsidaceous forms and ascomycetes characterize the population. The infrequency of aspergilli is thought to be due to their poor competitive ability. Eight species, Absidia cylindrospora, Penicillium notatum, Pestalotia cocculi?, Cylindrocarpon heteronema, Gliocladium roseum, Trichoderma viride, Paecilomyces marquandii, and Penicillium funiculosum were widespread in the area. Species diversity equaled that observed in mesic temperate communities. Less than one third of the isolates were cellulolytic. Phytopathogens were common, a feature characteristic of tropical populations. Thermophilous fungi averaged 33 per gram dry soil in the casuarina forest and increased to 943 per gram in the insolated soil of the coconut grove. Thermotolerant forms (94% of the isolates) were abundant and were principally species of Aspergillus and Chaetomium. Thermophilic fungi were rare and of the six species isolated only Chaetomiun britannicum was widespread. Four species, Ch. osmaniae, Ch. medusarum, Ch. sulphureum, and Thielavia arenaria, appear to be new records for western hemisphere soils.