AbstractIn this and three further papers 205 yeasts and yeast states of Basidiomycetes and presumed relatives were investigated comparatively on the basis of the carbohydrate (neutral sugars) pattern of purified cell walls, urease‐activity, diazonium blue B reaction on the production of extracellular amyloid compounds (EAS), fermentation of carbohydrates, and ubiquinone data. A clustering leading to theProtomyces‐, theMicrobotryum‐, theUstilago‐, theDacrymyces‐, and theTremella‐type became apparent, especially from the qualitative and quantitative cell wall carbohydrate pattern.The different yeast types correspond well with 5S rRNA clusters known from the literature. 31 strains clustering within theMicrobotryum‐type comprise the phragmobasidial smut fungi of dicotyledonous hosts (Microbotryum. Sphacelotheca), the phragmobasidialRhodosporidium‐ andLeucosporidium‐species including some anamorphRhodotorula‐species, which lack an oxidative degradation of myo‐inositol, the generaSporobolomycesandSporidiobolus, the Septobasidiales and some simple septate Auriculariales e.g.Agaricostilbum, Platygloea. Main characteristics of theMicrobotryum‐type are: 1. The absence of extracellular amyloid compounds. 2. The dominance of mannose and the presence of fucose as cell wall constituents. 3. A positive DBB‐reaction and splitting of urea. FourUstilagospecies parasitic on dicotyledonous hosts were transfered toMicrobotryum (M. scabiosae, M. scorzonerae, M. cordae, M. vinosum) as a consequence from cell wall carbohydrate composition, production of rhodotorulic acid, and 5S rRNA sequence data from the literature.The predominance of mannose in the cell wall — otherwise only known from ascomycetous yeasts –, a type A secondary structure of 5S rRNA, a simple unifactorial mating system in all parasitic smut species suggest that theMicrobotryum‐typemight be ancestral to theUstilago‐type. An evolution of simple (“siphonal”) holobasidia from “pseudotr