The phenotypic features of a clinical isolate of the new species Mycobacterium interjectum, identified on the basis of its 16S rRNA gene sequence, are compared with those of the type strain. The differentiation of M. interjectum from Mycobacterium gordonae or Mycobacterium scrofulaceum is not achievable on the basis of phenotypic traits usually tested for mycobacterial speciation, but it can be reached by 16S rRNA gene sequencing or by high performance liquid chromatography (HPLC) of cell wall mycolic acids. The former reveals sequence identity with the signature region of the type species, and the latter yields a profile which is easily differentiated from those of the other two species. The unique HPLC profile of M. interjectum is reported here for the first time and so are the MICs of a wide spectrum of drugs.