Endosymbiosis is a pervasive feature of eukaryotic life. In many cases, endosymbionts of eukaryotes are bacteria, but growing evidence indicates that many fungi also can occur as symbionts within eukaryotic hosts. Protists represent an exceptionally diverse array of eukaryotic life, and often, many diverse lineages of protists can be recovered from single sampling sites, where they engage in a variety of ecological roles. The goal of this research project is to investigate the phylogenetic diversity of fungi living in endosymbiosis with protists. I have made progress in three areas to date: (1) assessing the diversity of fungi encountered in water samples from a freshwater pond; (2) quantifying exogenous inputs of fungal propagules into that pond; and (3) developing methods for isolating pure cultures of protists and examining their fungal associates.