Although DNA sequencing method has not been used routinely for detecting resistance of Mycobacterium tuberculosis to antituberculosis drugs, it is suggested for the investigation of gene mutations causing resistance. Using DNA sequencing (Automated Applied Biosystems), we attempted to determine mutations in the 81-bp cor region (rifampin resistance determining region) of the rpoB gene in 48 Mycobacterium tuberculosis strains found to be rifampin resistant by classical phenotypic methods. Of the 48 strains, 46 (95.8%) were found to have rpo gene mutations with 13 different types while in two (4.2%) of the 48 strains, no mutations were detected. None of the strains had mutations at more than one codon. Point mutations at the 531st (52.1%) and 526th (18.9%) codons were frequent. The most frequent point mutation was Ser531Leu, and it was found in 21 (43,8 %) of 48 strains. This is the first study from Turkey, reporting Ser522Leu point mutation in one isolate and deletion of 515th codon (ATG - Met) in two isolates.