Two different bacterial strains with different drug susceptibilities were isolated from the sputum and an inflammatory discharge from a swelling in the left thigh of a patient with rheumatoid arthritis. Both bacterial strains were provisionally assigned to the genus Nocardia on the basis of their morphological and chemotaxonomic characteristics and were further studied in order to establish their taxonomic status. One strain (IFM 10034) was identified as Nocardia farcinica on the basis of its physiological characteristics. The other strain, which was designated Nocardia sp. strain IFM 10035(T), revealed a unique pattern of phenotypic properties that distinguished it from other representatives of established Nocardia species. Comparative 16S rRNA gene sequence studies of Nocardia sp. strain IFM 10035(T) also showed that the bacterium was closely related to the species Nocardia beijingensis. Determination of DNA-DNA relatedness, however, indicated that Nocardia sp. strain IFM 10035(T) could be delineated from N. beijingensis. The genotypic and phenotypic data combined indicated that the bacterium merits description as a new Nocardia species. The name proposed for the new species is Nocardia arthritidis sp. nov., the type strain being IFM 10035(T) (NBRC 100137(T), JCM 12120(T), DSM44731(T)). The present study suggests that Nocardia infections can be caused by multiple species of the bacterium.