Pucciniomycotina is a subphylum with a high diversity in terms of habitat and life history strategies that include plant parasites, animal associates (including opportunistic human pathogens), saprobes and antagonists of other fungi. The class Cystobasidiomycetes within this subphylum is a representative of such diversity and remains understudied. Their role in nature and the associations they establish with their hosts for most of the species is still unknown. In this study we used taxonomic and phylogenetic methods to present an inventory of strains in this class collected for more than 20 years from all over the world and preserved in the Aime Lab Culture Collection. Molecular and morphological data for six new species in the genera <i>Bannoa, Buckleyzyma</i>, <i>Halobasidium</i> and <i>Sakaguchia </i>were also presented. Additionally, mating experiments were performed by pairing strains of <i>Bannoa</i>; this is the second time in the genus that clamp connections and basidia are observed. We also found that newly described species of <i>Bannoa</i> can be co-inhabitants of sori of rust fungi. In general, antagonistic interactions can occur through: 1) direct physical contact between two fungi, i.e., mycoparasitism; or, 2) the production of antimicrobial compounds. In the Cystobasidiomycetes, direct physical antagonistic interaction which is associated with sexual states has been reported in species of Cystobasidium, Naohidea, Cyphobasidium and Occultifur. On the other hand, the production of antimicrobial compounds which mainly occurs between the yeast stage of the fungi and other organisms has only been reported in Cystobasidium pallidumand Hasegawazyma lactosa. We also hypothesize that the common ancestor to Cystobasidiomycetes is a mycoparasite due to the fact that this life strategy is present in most of the lineages in the class. To test this hypothesis, we grouped into five categories the host association or substrate from which strains of extant species in the Cystobasidiomycetes were isolated, i.e.,animals, plants, fungi, aquatic or decaying organic matter. We constructed a resolved phylogeny for the class based on seven locito study the evolutionary origins of mycoparasitism through ancestral character reconstruction with representation of all described species. Our analysis suggests that the most likely hypothesis is that the most recent common ancestor of the Cystobasidiomycetes was associated to fungi.<br>