Bacterial taxonomy based on phenotypic properties has encountered several problems: many organisms grow to poorly under laboratory conditions to be studied; the same phenotypic property often arises independently in more than one branch of a phylogenetic tree; and phenotypic schemes sometimes become unwieldy. Thus, molecular approaches have found a niche in taxonomy. Measurement of DNA hybridization between strains is the single most definitive tool for defining a species. Data on sequences of DNA and amino acids can be used to infer phylogeny. The molecular phylogenetic approach is most useful at levels ranging from kingdom to species. Restriction fragment length polymorphisms or variability in random polymerase chain reactions often differentiates strains. In the majority of cases, different molecular approaches tend to confirm established taxonomic schemes for bacteria. However, discrepancies between standard taxonomy and molecular data are not rare, especially when dealing with anaerobes. The contemporary approach to taxonomy takes into account both standard and molecular data.