Use of cefoperazone in a patient with Aeromonas caviae in the respiratory tract selected a mutant that constitutively produced β-lactamase. This mutant, in contrast to its parental strain with an inducible β-lactamase, showed enhanced resistance to newer cephalosporins and aztreonam. This observation suggested that species of Aeromonas, like those of other genera with inducible β-lactamases, may pose therapeutic problems associated with the rapid development of multiple β-lactam resistance. Thus, a study was designed to identify the β-lactamases in 12 strains representing four species of Aeromonas and assess their role in drug resistance. Eleven strains possessed inducible β-lactamases. One strain showed no detectable activity. An analysis of substrate and inhibitor profiles, isoelectric points, and β-lactam susceptibility patterns revealed the presence of at least four distinguishable inducible β-lactamases. These enzymes were involved in the resistance of strains within the genus to penicillins, cephalosporins, aztreonam, and imipenem but not cefoxitin. Unlike most other organisms with inducible β-lactamases, all four strains of A. caviae, one of four strains of A. sobria, and one of three strains of A. hydrophila possessed two distinct inducible β-lactamases. Furthermore, substrate and inhibitor profiles revealed that many of these Aeromonas β-lactamases were distinct from inducible enzymes that have been characterized in other genera of gram-negative bacteria.