New techniques for evaluating the biological diversity of naturally occurring microbial assemblages combine nucleic-acid-sequencing techniques with molecular-phylogenetic analysis. These molecular techniques avoid many of the selective biases inherent in traditional cultivation-based surveys and provide a universal criterion for identifying and relating diverse microbial species. Application of the molecular approach in marine environments has revealed the existence of unique and previously unrecognized microorganisms. These are providing fresh insight into the ecology, evolution and biotechnological potential of the largely untapped resource represented by the marine microbial world.