The International Committee on Systematic Bacteriology recommended that the category Candidatus, recently proposed by Murray and Schleifer (Int. J. Syst. Bacteriol. 44:174-176, 1994) to record the properties of putative taxa of procaryotes, should be implemented. This category should be used for describing procaryotic entities for which more than a mere sequence is available but for which characteristics required for description according to the International Code of Nomenclature of Bacteria are lacking. In addition to genomic information, such as sequences apt to determine the phylogenetic position of the organism, all information, including structural, metabolic, and reproductive features, should be included in the description of a provisional taxon, together with the natural environment in which the organism can be identified by in situ hybridization or other similar techniques for cell identification.