Modern approaches to microbial classification and identification, particularly those based on nucleic acid analysis, have raised the awareness and interest of microbiologists in systematics during the past decade. The extended scope of the subject has revolutionized microbial ecology with the demonstration of uncultivable microorganisms as a major component of the biosphere and evolution, with the ribosomal RNA phylogenetic tree as the basis of current classifications. However, advances in microbial systematics have also had enormous impact on other, diverse aspects of microbiology such as...