The resistance patterns and O-serotypes of 2952 Pseudomonas aeruginosa isolates were studied and the relationship between patterns and serotypes was investigated. The penicillinase-producing and cephalosporinase-overproducing phenotypes were significantly more frequent in intensive care units than other wards, but there was no difference for the intrinsic resistance phenotype. The predominant serotypes were O6, O11 and O1. The incidence of serotype O12 was low. O11 isolates were more common in intensive care units and more resistant to all antibiotics than other isolates. Most O12 isolates had a penicillinase-producing phenotype and were resistant to aminoglycosides and ciprofloxacin, but susceptible to fosfomycin.