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The Cuatro Cienegas basin (Coahuila, Mexico) is a composite of different water systems in the middle of the desert with unusually high levels of endemism and diversity in different taxa. Although the diversity of macrobiota has been well described, little is known about the diversity and distribution of microorganisms in the oligotrophic ponds. Here we describe the extent and distribution of diversity found in aquatic prokaryotic communities by analysis of terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes and phylogenetic analysis of cloned genes. Twelve locations within the basin were sampled. Among all the samples, we found a total of 117 operational taxonomic units (OTUs) using T-RFLPs, which ranged in any single sample from four to 49. OTU richness and Shannon diversity indices for different sites varied, but none were particularly high. 16S rRNA gene sequence data showed 68 different phylotypes among 198 clones. The most abundant phylotypes were Gamma- and Betaproteobacteria, and extreme halophiles. The differences among sites were significant; 45 TRFs were found only once, and 37% of the total diversity was represented by differences between sites, suggesting high beta-diversity. Further studies are needed to test whether this is a direct consequence of environmental heterogeneity in the basin.