Objective: To investigate ciprofloxacin and im ipenem in vitro and in vivo inducibility on Pseudomonas aeruginosa cross resistan ce.Methods：The cross resistance mutant strains in vitro and in vivo were selec ted with clinically isolated sensitive strain of P.aeruginosa PA5 by passage on MH agar containing different concentrations of ciprofloxacin or imipenem and experimental murine peritonitis.Results： The cross resistance rate of P.ae ruginosa selected by 1 or 2×MIC ciprofloxacin and imipenem were 9.2% and 20.8%. The 4 strains of cross resistance mutants were obtained when ciprofloxacin therapeutic dosage were 10～25mg/kg.The 2 strains of cross resistance mutants were ob tained when imipenem therapeutic dosage were 5～10mg/kg.Conclusion： T he cross resistance mutants for ciprofloxacin and imipenem could be induced and selec ted in vitro and in vivo with both agents.There is no difference in resistance mutation frequency and cross resistance rate between ciprofloxacin and imipenem in vitro.The in vivo cross resistance is related with dosage.