The name Agaricus subfloccosus (J. E. Lange) Pilát, based originally on Danish specimens, has been applied to mushrooms growing in coastal northwestern Europe as well as to mushrooms associated with Picea (and Abies) at higher elevations in western Europe. Corresponding populations are also found in these two habitat zones in western North America. Material of both lowland and highland forms from Europe and North America was studied using morphological, cultural, and genotypic approaches. Homothallism was consistently observed in those isolates studied. Individual and population level genetic data are also consistent with homothallic reproduction. Dissimilarity analysis of multilocus nuclear and mitochondrial genotypes based on allozyme and DNA RFLP markers provided strong evidence that, within either ecologically distinct group, the amphiatlantic populations were genotypically very similar. Conversely, there were large genotypic differences between the two ecologically distinct groups, whether within or between continents. Supported by cultural and morphological evidence, these data indicate that two ancient, phylogenetically distinct entities exist within current concepts of A. subfloccosus. A formal taxonomic resolution is complicated by the lack of a holotype for Lange's species.