The post antibiotic effect (PAE) of 8 strains of Pseudomonas aeruginosa with different susceptibility to sparfloxacin and ciprofloxacin was evaluated by the colony counting method using centrifugation to remove the antibiotic. The bacteria were exposed for 60 min to a quinolone concentration of 1 mg/ml. The MIC of sparfloxacin and ciprofloxacin ranged from 0.25-256 mg/ml and from 0. 25-128 mg/ml, respectively. PAE values ranged from 46 8.71 to 59.6 2.51 min and from 46.33 15.2 to 62.6 3.70 min, respectively. No significant statistical differences were found in the PAE duration for quinolones nor between strains. No correlation could be established between PAE and susceptibility of P. aeruginosa to quinolones.