The aim of this study was to determine the prevalence and antibiotic resistance rates of extended-spectrum β-lactamase (ESBL)-producing Proteus mirabilis strains isolated from inpatients at the Split University Hospital (southern Croatia) during a survey performed between 2005 and 2008. A total of 2152 consecutive isolates of P. mirabilis were isolated. The prevalence was 0.5 % in 2005 and increased significantly to 20.9 % by 2008. Strains were most frequently isolated from urine (36.5 %) and bronchial aspirates and wound swabs (11.3 %). ESBL-producing P. mirabilis isolates showed very high resistance rates to the majority of non-β-lactam antibiotics and were susceptible to a β-lactam/β-lactamase inhibitor and carbapenems. The isolates were genotyped and their ESBLs were molecularly characterized. Strains originating from the intensive care unit and the surgery and neurosurgery wards were clonally related. All P. mirabilis isolates produced the TEM-52 type of ESBL. To the best of our knowledge, our work detailed here and summarized in an earlier communication is the first report of such isolates from southern Croatia. Increased monitoring and screening for ESBL production in this species at our hospital is mandatory.