Extended spectrum beta-lactamase (ESBL) and ampicillinase C (AmpC) producing Enterobacteriaceae are nowadays frequently isolated in clinical practice. Carbapenems are generally the drugs of choice in such a case and resistance to these molecules is on the rise. Rationalizing their use is to be considered essential, possibly identifying alternative regimens. We thus examined 10 strains of Enterobacteriaceae isolated from patients previously unsuccessfully treated with a beta-lactam or a quinolone; eight strains were either ESBL or AmpC producers. Ulifloxacin showed minimum inhibitory concentrations (MICs) lower than ciprofloxacin and levofloxacin. Tested with the checkerboard method, the association of ulifloxacin and piperacillin/tazobactam proved fully synergistic on five strains and partially synergistic on three. The above association was fully synergistic towards three strains resistant to piperacillin/tazobactam and one strain resistant to ulifloxacin, with MICs in association easily obtainable at standard doses. Our in vitro study demonstrates a synergistic activity of ulifloxacin and piperacillin/tazobactam in association towards ESBL and AmpC-producing Enterobacteriaceae. Clinical studies are needed to confirm in vivo the effectiveness of this regimen.