Nucleotide sequences of genes conferring isoniazid resistance (katG, inhA, oxyR-ahpC and ndh) and ethionamide resistance (ethA) in 160 drug-resistant Mycobacterium tuberculosis clinical isolates from Thailand were analysed. Mutations in the katG gene were found in 129 isolates, predominantly at codon 315, which was mutated in 127 isolates. Twenty-two isolates had mutations in the inhA promoter and coding region. Mutations in the oxyR-ahpC intergenic region and in ndh were detected in four and one isolate(s), respectively. Of 24 ethionamide-resistant isolates, 13 had mutations in the ethA gene. However, these mutations were dispersed along the entire gene, with no codon predominating significantly.