The effect of fluoroquinolones in Citrobacter freundii strains that results in a decreased expression of cephalosporin-hydrolysing β-lactamases was studied. Resistance to broad-spectrum cephalosporins and penicillins in two C. freundii clinical isolates was associated with moderate production of chromosomal AmpC-type-β-lactamase in addition to changes in the outer membrane proteins profile with respect to wild-type C. freundii strains. Ten quinolone-resistant mutants were derived from the two clinical isolates using increasing fluoroquinolone concentrations. The level of susceptibility to cephalosporins and meropenem of these 10 mutants was increased and was associated with a 3.6-32% diminution in the hydrolyzing activity of their periplasmic extracts containing β-lactamases on cephaloridine as compared with those from their parent strains. Susceptibility to cephalosporins and meropenem, as well as the expression of chromosomal AmpC-type-β-lactamase in C. freundii strains, was influenced by the exposure to quinolones.