Aims: Resistance to fluoroquinolones is partially the result of a decrease in drug accumulation in Escherichia coli through different mechanisms. However, the variable contribution of these mechanisms with respect to different fluoroquinolones is poorly investigated. Therefore, the current study aimed to compare the contribution of resistance attributed to efflux-mediated mechanisms for different fluoroquinolones. Methods and Results: Susceptibility of enrofloxacin, marbofloxacin and ciprofloxacin were compared after treatment with an efflux pump inhibitor in 17 ciprofloxacin-resistant E. coli isolates, and also the expression profile of the genes encoding the porins and efflux pumps involved in this resistance was evaluated. After treatment with the efflux pump inhibitor Phe-Argb-naphthylamide (PAbN), susceptibilities differed significantly between antimicrobial agents, the decrease for MIC being higher for enrofloxacin than for marbofloxacin or ciprofloxacin. AcrB expression level increased significantly (+26%) in ciprofloxacin-resistant E. coli isolates compared with ciprofloxacinsusceptible isolates, whereas the expression level decreased for ompF (À50%) and ompC (À30%). Conclusions: There was a higher contribution of resistance nodulation division (RND) efflux pumps to resistance to hydrophobic fluoroquinolones. Significance and Impact of the Study: Comparison between expression profile of efflux pumps and hydrophobicity of the antimicrobial agents could result in variable resistance for different fluoroquinolones.