Following recent advances in medical technology, the increased number of immunocompromised patients such as those with organ transplants has led to an increase in opportunistic infections due to Nocardia. Although nocardiosis has been considered to be rare, recent reports indicate that the incidence of the infection is increasing. The Nocardia asteroides group is the principal source of infectious species, but the definition of species in Nocardia is problematic because species identification of N. asteroides has been based on mainly morphological and biochemical properties. Additionally, it was assumed that a clinical strain with properties that did not fully coincide with existing species was N. asteroides sensu lato. This caused problems in both clinical and taxonomical fields, and reclassification of N. asteroides sensu lato was necessary. Therefore, determination of the appropriate taxonomic position of N. asteroides sensu lato that is now classified as N. asteroides sensu stricto was conducted using a molecular phylogenetic method. From 1965 to 2001, twenty-two strains of N. asteroides sensu lato were isolated from clinical samples. The phylogenetic tree using 16S rDNA sequences and detailed biochemical characters on the 22 isolates was determined. Results revealed that nine strains should be reclassified into species other than N. asteroides sensu stricto, and we proposed nine new species of the genus Nocardia. In addition, we proposed eight other new species of the genus Nocardia from other samples; hence seventeen new species were proposed in total. We also reported the first infectious cases due to Nocardia beijingensis, Nocardia transvalensis and Nocardia pseudobrasiliensis in Japan.