TEM studies show that species of Taphrina and Protomyces have cell walls with a thick, electron-transparent inner layer, similar to those of the Endomycetales, e.g., of species of Saccharomyces or Hanseniaspora. The budding is holoblastic at the attenuated end or ends of the cell and is rather similar to that of the apiculate yeasts. Members of the genera Microstroma and Exobasidium have an electron opaque, often multilayered cell wall and show enteroblastic budding, similar to that of the "red yeasts" (Sporobolomycetales) and Ustilaginales. The Taphrinales are consequently classified in the Endomycetes (Ascomycotina), while Microstroma and Exobasidium are placed in the Basidiomycetes. Species of the genera Kabatiella and Aureobasidium have ascomycete-like cell walls, and are therefore not related to Microstroma. The classes of the Ascomycotina and Basidiomycotina are discussed briefly. The Taphrinales are usually considered to be primitive Ascomycetes and have, as a rule, been classified in the Endomycetes (=Hemiascomycetes), which mainly comprise the ascus-forming yeasts. Some authors, however, have suggested that the Taphrinales are related to the Basidiomycetes, particularly to the Sporobolomycetales, Exobasidiales, and Uredinales. As early as 1884, Sadebeck observed that the asci of Taphrina species may form exogenous, stalked budding cells resembling basidiospores. Lohwag (1934) considered the ascus of Taphrina to be a degenerate basidium. Graafland (1960) compared Taphrina to Exobasidium, drawing particular attention to their biological similarity. Both genera contain plant parasites causing hyperplasia, chlorosis, galls or witches' broom, mainly on broad-leaved trees and shrubs. All species of both genera have a parasitic and a saprophytic state. In the parasitic state intercellular hyphal structures are formed which perforate the cuticle or emerge through the stomata giving rise to superficial hymenia of asci or basidia (FIGS. 1, 2). The saprophytic state is yeast-like and propagates by budding. Only young, growing structures of the host plant such as sprouting buds and germinating seeds are susceptible to infection. The species of the genus Microstroma Niessl have a yeast-like, saprophytic and a mycelial parasitic state. The genus had been classified in the Hyphomycetes until Blanz (1978) and Oberwinkler (1978) recognized its basidiomycetous affinity. Microstroma has often been connected with Kabatiella Bubak and Aureobasidi