A Gram-positive, aerobic, spherical actinobacterium, designated strain 4-0(T), was isolated from a medieval wall painting and characterized to determine its taxonomic position. The peptidoglycan of strain 4-0(T) was of type A4alpha, with lysine as the diagnostic cell wall diamino acid and an interpeptide bridge of Lys-Gly-Glu. Its quinone system contained predominantly MK-9(H2) (64%) and its polar lipid profile consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol, four unknown glycolipids, two unknown phospholipids and an unknown lipid. The fatty acid profile of strain 4-0(T) was represented by significant amounts of ai-C15:0 and moderate amounts of ai-C17:0, i-C16:0 and i-C15:0 fatty acids. Spermidine was predominant in the polyamine pattern. The G+C content of the genomic DNA was 68 mol%. Comparative 16S rDNA sequence studies revealed the highest similarity values (95.4-96.1%) between strain 4-0(T) and species of the genus Micrococcus and certain species of the genus Arthrobacter, including Arthrobacter pascens DSM 20545(T), Arthrobacter ramosus DSM 20546(T), Arthrobacterprotophormiae DSM 20168(T), Arthrobacternicotianae DSM 20123(T) and Arthrobacter globiformis DSM 20124(T). Phylogenetic analyses demonstrated that strain 4-0(T) branches deeply on the Micrococcus lineage. Because it is almost equidistant phylogenetically from the genera Micrococcus and Arthrobacter and possesses significant differences in chemotaxonomic characteristics from members of these genera, it is suggested that strain 4-0(T) be classified as a novel species in a new genus; the name Citricoccus muralis gen. nov., sp. nov. is proposed. The type strain is strain 4-0(T) (= DSM 11442(T) CCM 4981(T)).