Objectives: For fluoroquinolones, the area under the free plasma concentration-time curve divided by the MIC (fAUC/MIC) best predicts bacterial killing in mice and outcomes in patients. However, it is unknown whether the shape of the antibiotic concentration profile affects resistance emergence. Our objective was to compare killing and resistance between ciprofloxacin concentration profiles with different shapes at the same fAUC/MIC and identify the durations of ciprofloxacin exposure that minimize resistance emergence. Methods: Static time-kill studies over 24 h using Pseudomonas aeruginosa ATCC 27853 assessed fAUC/MIC of 44 and 132 of ciprofloxacin (MICCIP=0.25 mg/L) and fAUC/MIC of 22, 44 and 132 of ciprofloxacin plus an efflux pump inhibitor (MICCIP+EPI=0.031 mg/L) at initial inocula of 104, 105 and 106 cfu/mL. Ciprofloxacin was added at 0 h and rapidly removed at 1, 4, 10, 16 or 24 h. Mutant frequencies and MICs were determined at 24 h. Results: High ciprofloxacin concentrations over 1-10 h yielded more rapid and extensive initial killing compared with 16 and 24 h exposures at the same fAUC/MIC. No resistance emerged for 1-10 h exposures, although regrowth of susceptible bacteria was extensive. Ciprofloxacin exposure over 24 h yielded less regrowth, but ciprofloxacin-resistant bacteria at 5?MIC amplified by over 5 log10 and almost completely replaced the susceptible bacteria by 24 h; MICs increased 4-to 8-fold. Resistance also emerged on 3? MIC, but not 5? MIC, plates when efflux was inhibited.