Sensitivity of some bacteria (Aeromonas hydrophila, Pseudomonas fluorescence, P. anguilliseptica, P. aeroginosa and E. coli) to some antimicrobials was studied in vitro. Theses bacteria were highly sensitive to ciprofloxacin, danofloxcin and cephazon, while they were resistant to ampicillin and orbenin. Fish were experimentally infected with the tested bacteria. After the development of clinical signs of experimental infection. Fish were treated successively by using ciprofloxacin intraperitoneally at dose of 20 mg/kg body weight for three successive days. Pseudomonas fluorescence infected fish group was chesen to study the pharmacokinetics of ciprofloxacin after first dose and tissue residues after multiple doses (3 days). The kinetics of ciprofloxacin showed distribution half (t 0.5 (a b)0.41+-0.02 and 0.61+-0.03 and elimination half life to t0.5 el was 6.9+-0.41 and 5.7+-0.37 hrs in normal and infected fish respectively. The clearance of ciprofloxacin from all tissues was 120 hrs following treatment. High tissue residues of drug were recorded in kidney, liver and muscles respectively and were higher in infected fish than healthy one.