The complete nucleotide sequences of the Mycobacterium leprae 23 S and 5 S rRNA genes and their flanking regions are presented. As compared to other eubacterial homologous molecules the 23 S rDNA exhibits two insertions. A 16 nucleotide long insertion is almost unique to members of the genus Mycobacterium, while the second represents an extended version of helix 54. The potential or both insertions to serve as target for diagnostic oligonucleotide probes was proven by comparative sequence analysis of 23 S rRNA or several Mycobacterium species and by dot blot hybridization. In addition, a 19-mer oligonucleotide probe is described, which can be considered genus Mycobacterium-specific.