Rhizopogon is a hypogeous fungal genus that forms ectomycorrhizae with genera of the Pinaceae. The greatest number and species of Rhizopogon are found in coniferous forests of the Pacific Northwestern United States, where members of the Pinaceae are also concentrated. Rhizopogon spp. are host-specific primarily with Pinus spp. and Pseudotsuga spp. and thus are an important component of these forest ecosystems. Rhizopogon includes over 100 species; however, the systematics of Rhizopogon have not been well understood. Currently the genus is placed in the Boletales, an order of ectomycorrhizal fungi that are primarily epigeous and have a tubular hymenium. Suillus is a stipitate genus closely related to Rhizopogon that is also in the Boletales and host specific with Pinaceae. I examined the relationship of Rhizopogon to Suillus and other genera in the Boletales. Infrageneric relationships in Rhizopogon were also investigated to test current taxonomic hypotheses and species concepts. Through phylogenetic analyses of large subunit and internal transcribed spacer nuclear ribosomal DNA sequences, I found that Rhizopogon and Suillus formed distinct monophyletic groups. Rhizopogon was composed of four distinct groups; sections Amylopogon and Villosuli were strongly supported monophyletic groups. Section Rhizopogon was not monophyletic, and formed two distinct clades. Section Fulviglebae formed a strongly supported group within section Villosuli. Taxonomic revisions were proposed. Suillus, Truncocolumella, and the Gomphidiaceae were transferred to the Rhizopogonaceae. In Rhizopogon, sections Amylopogon, Rhizopogon, and Villosuli were elevated to subgenera. Subgenus Roseoli was erected to accommodate the second section Rhizopogon Glade. In section Fulviglebae, Stirps Vinicolor, Rhizopogon ochraceisporus, R. subclavitisporus, and R. clavitisporus were transferred to subgenus Villosuli while the remaining species in section Fulviglebae were transferred to subgenus Rhizopogon. / Graduation date: 1999