A. mellea consists of at least 10 reproductively isolated groups, the equivalent of biological species. Each biological species possesses bifactorial heterothallism with compatibility discernible by the gross mycelial morphology of paired monosporous isolates rather than the presence or absence of clamp connections and dikaryotic cells. Monosporous testers were obtained from 97 fruiting bodies in North America. Pairings of testers from different fruiting bodies indicated that each isolate belongs to 1 intersterile group, i.e., intersterility between groups is complete. Nutritional selection applied to confronted auxotrophic strains from 2 of the biological species revealed no prototrophy (genetic complementation) between the strains of these groups, whereas prototrophy was revealed by the same method within groups. Members of several of the biological species are distributed widely in North America. Isolates may be collected from a broad range of host species or also as saprophytes. The 10 biological species are not clearly distinguishable by unique geographical ranges or substrate specificities. A. mellea is a complex of morphologically distinct species. The taxon is apparently divided into genetically isolated distinct biological species.