Objective To investigate the expression of mar operon in clinical isolates of Escherichia coli. Methods mar operon was amplified by polymerase chain reaction (PCR) and the PCR products were analyzed by restriction endonuclease and identified by Southern blot. The expression of mar operon was determined by reverse transcription polymerase chain reaction (RT PCR). Results No marO ,marR , marA deficiency was found in all clinical isolates of Escherichia coli. The level of expression of mar operon in Mar strain was significantly higher than those in other strains, and increased significantly when incubated with sodium salicylate or tetracycline.Conclusions High expression of mar operon is responsible for the Mar phenotype of clinical strains of Escherichia coli and some mutations leading to high expression of mar operon may exist in marR and/or marO. The expression of mar operon can be induced by salicylate and tetracycline.