Thirty-seven multidrug-resistant and 13 pan-susceptible isolates of Mycobacterium tuberculosis were analysed for the diversity of genotypes associated with known drug-resistance mechanisms. The isolates were obtained from patients attending a university tuberculosis clinic in Monterrey, Mexico. A total of 25 IS6110-RFLP patterns were obtained from the multidrug-resistant tuberculosis (MDR-TB) isolates. Approximately 65% of the MDR-TB isolates were attributed to secondary resistance. Different drug-susceptibility patterns were seen with the clustered isolates. The percentage of isolates resistant to isoniazid (INH), rifampicin (RIF), ethambutol (EMB) and streptomycin (STR) was 100, 97.3, 48.7 and 67.6, respectively. The most common resistance-associated polymorphisms for the four drugs were as follows: INH, Ser315Thr (67.6%) in katG; RIF, Ser450Leu (41.7%) in rpoB; EMB, Met306Ile/Val/Leu (66.7%) in embB; and STR, Lys43Arg (24%) in rpsL. Drug-resistance-associated mutations were similar to changes occurring in isolates from other areas of the world, but unique, previously unreported, mutations in katG (n=5), rpoB (n=1) and rrs (n=3) were also identified.