The aim of this study was to investigate the frequency, location and type of katG mutations in Mycobacterium tuberculosis strains isolated from patients in Belarus. Forty two isoniazid-resistant isolates were identified from sputum of 163 patients with active pulmonary tuberculosis. Drug susceptibility testing was determined by using CDC standard conventional proportional method and BACTEC system. Standard PCR method for detection of isoniazid resistance associated mutations was performed by katG gene amplification and DNA sequencing. Most mutations were found in katG gene codons 315, 316 and 309. Four types of mutations were identified in codon 315: AGC-->ACC (n=36) 85%, AGC-->AGG (n=1) 2.3%, AGC-->AAC (n=2) 4.7%, AGC-->GGC (n=1) 2.3%. One type of mutation was found in codon 316: GGC-->AGC (n=18) 41.4%, four types of mutations were detected in codon 309: GGT-->GGT (n=7) 16.1%, GGT-->GCT (n=4) 9.2%, GGT-->GTC (n=3)6.9%, GGT-->GGG (n=1) 2.7%. The highest frequency of mutations sharing between primary and secondary infections was found in codon 315.