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Filamentous nonheterocytous cyanobacteria have been recognized as key ecosystem players in terms of soil stabilization, carbon sequestration, and water retention. Although the ecological importance of this group is well understood, its taxonomy is understudied. Microcoleus, Schizothrix, and Leptolyngbya, the most common cyanobacterial genera reported from soils, have all been found to be polyphyletic and in need of revision. In this study, strains morphologically corresponding to M. steenstrupii were isolated from North American desert soils. This species has been found to be polyphyletic, forming several distinct clades in numerous analyses. Three desert soil strains were analysed using ecological and geographical information, detailed morphological investigations, and DNA sequence data of the 16S rRNA gene and associated 16S-23S internal transcribed spacer. On the basis of this polyphasic characterization, we concluded that they represented the genus Symplocastrum. Until now, this cyanobacterial genus was poorly understood and lacked molecular definition. A more precise description of Symplocastrum is provided, including its phylogenetic position based on 16S rRNA gene sequence. Two new species are described (S. flechtnerae, S. torsivum) and Schizothrix californica is transferred to Symplocastrum. Phylogenetic placement of Symplocastrum is a critical component of the revisionary work underway in the nonheterocytous filamentous cyanobacteria and will lead to further revisionary work in this problematic group. Furthermore, with increasing taxonomic clarification of these filamentous taxa, ecological studies will have greater resolution and accuracy.