Arthrobacter sp. strain NRRL B-3381T (T = type strain) is a nonmycelial, nonsporulating actinomycete that produces the macrolide antibiotic erythromycin. This bacterium differs in many ways from the type species of the genus Arthrobacter (Arthrobacter globiformis), suggesting that a taxonomic revision is appropriate. The G + C content of strain NRRL B-3381T DNA is 71 to 73 mol%, and the peptidoglycan of this organism contains LL-diaminopimelic acid. Evolutionary distance data obtained from 16S rRNA sequences identified NRRL B-3381T as the deepest branching member of the Nocardioides group of actinomycetes. The principal long-chain fatty acids which we identified that distinguished strain NRRL B-3381T from related G + C-rich bacteria were 10-methyloctadecanoic (tuberculosteric), octadecenoic, and hexadecanoic acids. These characteristics, together with phage typing and biochemical characteristics, form the basis for our recommendation that strain NRRL B-3381 should be the type strain of a new taxon, for which we propose the name Aeromicrobium erythreum.