A clinical strain of Acinetobacter baumannii (strain Ab41) that was resistant to all β-lactam antibiotics tested except ceftazidime, ceftriaxone, ceftizoxime, and imipenam produced three β-lactamases: a presumptive chromosomal cephalosporinase, a TEM-1-like β-lactamase (pI 5.4), and a novel OXA-derived β-lactamase named OXA-21 (pI 7.0). The gene encoding OXA-21 was located in an integron. The nucleotide sequence showed three mutations compared with the sequence of OXA-3, with two being silent; the nonsilent mutation generated a substitution of Ile-217 to Met.