Specific questions regarding the community structure of VA-mycorrhizal fungi and the population biology of an ectomycorrhizal Basidiomycete, Suillus granulatus, were addressed. The distributional ecology of VAM fungal communities with grasses was studied across a climatic gradient in the hyper-arid central Namib dune field. VAM fungal communities were primarily structured by substrate stability and moisture availability. Five VAM species were found throughout the study area and were not host specific. Percent mycorrhizal colonization was correlated with moisture availability, whereas spore abundance was correlated with substrate stability. Moisture availability was the key factor influencing VAM fungal phenology: growth, assessed as increased colonization of roots, continued as long as moisture was available, and spore production occurred in response to declining moisture availability. While abiotic factors determine community structure of VAM fungi in the Namib dune field, preliminary studies suggest that the phytobiont mediates fungal response to these abiotic factors. Genetic analyses of S. granulatus single spore isolates using RAPD markers showed that a post-meiotic mitosis in the basidium produces heterokaryotic spores. Secondary homothallism provides an effective means for long distance dispersal, and may account for the broad geographic range of this ectomycorrhizal fungus. Secondary homothallism contributed to the failure of somatic incompatibility tests to delineate the spatial distribution of individuals in a natural population of s. granulatus. Analysis of genetic relatedness using RAPD markers demonstrated conclusively that somatically compatible individuals were not necessarily genetically identical. I concluded that RAPD marker analysis provides a more effective means for determining clonal distribution in ectomycorrhizal populations, than does somatic incompatibility testing. / Ph. D.