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Archaea are common and abundant members of biological soil crust communities across large-scale biogeographic provinces of and North America. Regardless of microbial community development, archaeal populations averaged 2 x 10(7) 16S rRNA gene copies per gram of soil, representing around 5% of the prokaryotic (total calculated bacterial and archaeal) numbers assessed by quantitative-PCR. In contrast, archaeal diversity, determined by denaturing gradient gel electrophoresis fingerprinting and clone libraries of 16S rRNA genes, was very restricted. Only six different phylotypes (all Crenarchaea) were detected, three of which were very dominant. Some phylotypes were widespread, while others were typical of Southern desert areas. (C) 2009 Elsevier Ltd. All rights reserved.