Nine yellow-pigmented, spherical bacterial strains isolated from a medieval wall painting (strain D7), from indoor air (strains 3, 6, 7, 13C2, 38, 83 and 118) and from an activated-sludge plant (strain Ballarat) were classified by a polyphasic approach. Analyses of the 16S rRNA gene sequences of three representatives (strains D7, 118 and Ballarat) indicated that they all belong to the genus Micrococcus. The three isolates shared the highest sequence similarities with Micrococcus luteus DSM 20030T (97.9-98%), Micrococcus antarcticus AS 1.2372T (97.9-98.3%) and Micrococcus lylae DSM 20315T (97.5-97.9%). DNA-DNA reassociation studies clearly demonstrated that all nine isolates belong to the species M. luteus. However, neither their chemotaxonomic features nor their physiological and biochemical properties were consistent with those of M. luteus DSM 20030T. In contrast to M. luteus DSM 20030T, all isolates investigated possessed MK-8(H2) as the major respiratory quinone, and strain Ballarat had an A4alpha peptidoglycan type. On the basis of analyses of their Fourier transform-infrared spectroscopy spectra, isolates D7, 3, 6, 7, 13C2, 38, 83 and 118 could be grouped into a single cluster separate from M. luteus DSM 20030T, strain Ballarat and M. lylae DSM 20315T. In addition, all these isolates could be distinguished from M. luteus DSM 20030T by their ability to assimilate D-maltose, D-trehalose, DL-3-hydroxybutyrate, DL-lactate, pyruvate and L-histidine and to hydrolyse casein. Strains D7, 3, 6, 7, 13C2, 38, 83 and 118 differed from both M. luteus DSM 20030T and strain Ballarat by their ability to assimilate acetate, L-phenylalanine, L-serine and phenylacetate. Furthermore, REP-PCR fingerprinting yielded one common band for these strains, whereas this band was not observed for M. luteus DSM 20030T, strain Ballarat or M. lylae DSM 20315T. On the basis of these data, the species M. luteus can be divided into three biovars that are distinguished by several chemotaxonomic and biochemical traits: biovar I, represented by M. luteus DSM 20030T; biovar II, represented by strains D7 (= DSM 14234 = CCM 4959), 3, 6, 7, 13C2, 38, 83 and 118; and biovar III, represented by strain Ballarat (= DSM 14235 = CCM 4960). On the basis of the results generated in this study, emended descriptions of the genus Micrococcus and the species M. luteus and M. lylae are given.