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A distinctive variety of A. bisporus has been discovered in several habitats in the low Sonoran Desert of the Coachella Valley of California at elevations below sea level. The tetrasporic character of the basidia is consistently predominant in this local population. The spores of this population tend to be shorter and narrower than those of the bisporic strains. The genotypes of two individuals from this population clearly place them in the species A. bisporus, where they form a cohesive group in species-wide cluster analyses of nuclear genotypic similarity. The relative genetic divergence of this population from others in the species is of a similar order to that observed between other geographically isolated, bisporic populations of the species. The tetrasporic individuals exhibit complete interfertility with several bisporic strains of European and Coastal Californian origin. Because of their basidial and spore morphologies, these mushrooms do not fall within the traditional circumscription of A. bisporus. Varietal status is proposed for these distinctive, tetrasporic members of the species, and A. bisporus var. burnettii is described. Primary habitat of these mushrooms is being destroyed rapidly near our field sites. Geographical range of the variety is unknown.