Objective(s): To date, the most important genes responsible for tetracycline resistance among Acinetobacter baumannii isolates have been identified as tet A and tet B. This study was carried out to determine the rate of resistance to tetracycline and related antibiotics, and mechanisms of resistance. Materials and Methods: During the years 2010 and 2011, a total of 100 A. baumannii isolates were recovered from patients in different hospitals of Tehran, Iran. Antimicrobial susceptibility to tetracycline, minocycline, doxicycline and tigecycline was evaluated by E-test. Polymerase chain reaction (PCR) of the tet A and tet B genes was performed using specific primers, after which the isolates were subjected to Repetitive Extragenic Palindromic-PCR (PCR) to identify the major genotypes. Results: Of all isolates, 89 were resistant to tetracycline (MIC50 = 32 mu g/ml, MIC90 = 512 mu g/ml). Minocycline with the resistant rate of 35 (MIC50 = 16 mu g/ml, MIC90 = 32 mu g/ml) and doxicycline with the resistant rate of 25 (MIC50 = 16 mu g/ml, MIC90= 32 mu g/ml) have a good activity against A. baumannii isolates. All isolates were sensitive to tigecycline. Frequencies of tet B and tet A genes and coexistence of tet A and tet B among the isolates resistant to tetracycline, were 87.6, 2.2 and 1.1, respectively. Distribution of REP-types among A. baumannii isolates was types A (40), B (30), C (10), D (5) and E (5). Conclusion: It seems that tet A and tet B genes play an important role in the induction of resistance towards tetracyclines used in this study. It is suggested that further studies focus on other antimicrobial drugs and combinations in order to achieve a successful therapy against multi drug resistance (MDR) A. baumannii strains in Iran
