Analysis of small subunit rRNAs retrieved from mixed microbial populations has provided a framework for understanding global microbial diversity (1). These molecular phylogenetic analyses have revealed the presence of previously undetected species and lineages in a wide variety of microbial habitats, and placed newly discovered microbial groups on a universal phylogenetic map. These studies also show that some microbial groups, undetected by traditional cultivation approaches, are major components of natural microbial communities. Whole genome sequencing, developed over the past several years, has been so successful that more than 40 prokaryotic genome projects have either been completed or are under way. New genome technologies also offer novel avenues for characterization of uncultivated prokaryotes. Since the main requirement for such an approach is the availability of high-quality genomic DNA (readily isolated from environmental samples), large genomic fragments of uncultivated organisms from mixed populations can, in theory, be isolated and characterized using developing genomic techniques. Application of “environmental genomits” could provide access to gene organization, evolution, and physiological potential of novel uncultivated microorganisms. The archived genomes of uncultivated microbial types also become reagents, which could be used for ex-