Phylogenetic evidence derived from a 16S ribosomal DNA analysis indicated that the type strain of Micrococcus agilis, DSM 20550 (= ATCC 966 = CCM 2390), is less closely related to the type species of the genus Micrococcus, Micrococcus luteus, than to the type species of the genus Arthrobacter, Arthrobacter globiformis, and related Arthrobacter species. The phylogenetic position of M. agilis is supported by the presence of peptidoglycan variation A3 alpha and by the presence of MK-9(H2) as the major isoprenolog, a characteristic also found in strains of A. globiformis, Arthrobacter crystallopoietes, Arthrobacter atrocyaneus, Arthrobacter citreus, Arthrobacter aurescens, Arthrobacter ilicis, Arthrobacter ureafaciens, Arthrobacter oxydans, Arthrobacter histidinolovorans, and Arthrobacter nicotinovorans. The last six species and M. agilis are characterized by the presence of threonine in the interpeptide bridge of the peptidoglycan. Threonine has not been found in the peptidoglycans of other Arthrobacter species or in members of the genus Micrococcus. Despite the fact that a morphological life cycle is not known, these data support the proposal that M. agilis should be transferred to the genus Arthrobacter as Arthrobacter agilis comb. nov.