The Actinomycetales are an order of Actinobacteria. A member of the order is often called an actinomycete. The actinomycetes are very diverse and contain a variety of subdivisions, as well as yet-unclassified isolates, mainly because some genera are very difficult to classify because of a highly niche-dependent phenotype. For example, Nocardia contains several phenotypes first believed to be distinct species before their differences were shown to be entirely dependent on their growth conditions.