Four nocardioform bacterial strains isolated from clinical respiratory sources were characterized using a polyphasic taxonomic approach. On the basis of 16S rRNA gene sequence analyses, these strains were found to be 100 % similar to each other and were shown to belong to the genus Nocardia. Chemotaxonomic data [major menaquinone: ω-cyclic isoprene side chain MK-8(H₄(cycl)); major polar lipids: diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol and phosphatidylinositol mannosides; major fatty acids: monounsaturated fatty acids with a considerable amount of tuberculostearic acid; and mycolic acids (52-62 carbon atoms)] were consistent with the assignment of the novel strains to the genus Nocardia. Comparative phylogenetic analysis of the 16S rRNA gene sequences showed that the novel strains were related to Nocardia cerradoensis DSM 44546(T) (99.8 %) and Nocardia aobensis JCM 12352(T) (99.6 %). Analysis of gyrB gene sequences showed these strains were related to N. aobensis (96.6 %) and to N. cerradoensis (96.3 %). The results suggest that gyrB gene sequencing is a more powerful tool than 16S rRNA gene sequencing for taxonomic identification within the genus Nocardia. DNA-DNA hybridization and physiological and biochemical tests supported the genotypic and phenotypic differentiation of the novel strains from related species. These data indicated that the new strains represent a novel species within the genus Nocardia, for which the name Nocardia mikamii sp. nov. is proposed, with strain W8061(T) (=DSM 45174(T)=JCM 15508(T)) as the type strain.