Analysis of the monosaccharide composition of purified cell walls of unicellular and filamentous ascomycetous fungi shows three patterns: (1) the mannose glucose type (for most hemiascomycetous yeasts) (2) the mannose glucose galactose type (for several members of all three main ascomycetous clades) and (3) the mannose glucose galactose rhamnose type (for members of the Euascomycetes and the Protomyces/ Schizosaccharomyces group).

In order to estimate the usefulness of the carbohydrate patterns for phylogenetic analysis we compared them with a phylogenetic tree based on 18SrRNA-gene sequences using the Neighbor-Joining Method. In contrast with the situation for basidiomycetous fungi, the Ascomycota show no fixed cell wall type for the three classes. Based on cell wall carbohydrates, sequence data and molecular characters the Hemiascomycetes appear as the first branch within the Ascomycota. A second clade, comprising the genera Schizosaccharomyces, Pneumocystis, Taphrina, Protomyces, Neolecta and Saitoella, appears as a sister group of the Euascomycetes. We discuss the erection of a new class for this group of ascomycetous fungi for which we propose the name Protomycetes.