The present invention relates to a method for diagnosis, identification and characterization of M. tuberculosis or any other mycobacteria by using PCR and shift mobility assay (SMA). The method is based both on microheterogeneities observed within the 16S rRNA sequences established for mycobacteria and on nucleotide gaps and mismatches which cause a decrease in the electrophoretic mobility of DNA heteroduplex in polyacrylamide gels. The PCR strategy is based on the use of a specific primers for mycobacteria genera and divergence in sequences found in 16S rRNA coding gene. The difference in nucleotide sequence was enough to identify mycobacteria species, since a remarkable shift between single stranded and homoduplex bands in PAGE were observed among mycobacteria tested, when Mycobacterium tuberculosis was used as standard. The shift of PCR product of bacteria other than mycobacteria was observed above the single stranded band in PAGE, both in standard cultures and clinical specimens. SMA can thus provide a fast and sensitive method for detection and classification of mycobacteria in clinical samples as well as pure culture.