OBJECTIVE: Recently a publication of Brenner et al. introduced 11 genetically distinct species within the genus Citrobacter. These newly recognized Citrobacter species can be classified by means of their biochemical characteristics. The aim of this study was to examine the distribution and susceptibility of Citrobacter isolates in our patient population. METHODS: A total of 126 samples---containing a Citrobacter species---was collected from 116 hospitalized patients during a 6-month period. Organisms were identified according to standard procedures. Antimicrobial susceptibility testing was performed by agar dilution on Mueller-Hinton agar, and interpretation was based on NCCLS criteria. RESULTS: C. freundii was the most common organism isolated (n=59), followed by C. braakii (n=25) and C. koseri (n=23). The urinary tract and the respiratory tract were found to be the predominant sites of colonization or infection, accounting for 45% and 32% of all isolates respectively. It appeared that young children (<12 months old) and the elderly were most at risk of acquiring Citrobacter. Two-thirds of all specimens contained other organisms in addition to Citrobacter. Most Citrobacter isolates were related with a predisposing factor. Species-related differences were found in the susceptibility pattern. CONCLUSIONS: These findings suggest that citrobacteria are important opportunistic pathogens contributing to colonization or infection in our hospital population.