Different kind of mineral waters used for drinking purposes or hydromineral therapy and surface environmental waters for recreational use were tested for the presence of Pseudomonas aeruginosa. A comparison of antibiotic resistance patterns and serotyping between water strains and P. aeruginosa isolates from different infection sites of hospitalized humans (Pisa General Hospital) was performed. Altogether one hundred and fifty-six strains were characterized. All isolates were most resistant to furans. A very high degree of resistance to amoxicillin, clavulanic acid, ampicillin, chloramphenicol, tetracycline and trimethoprim was pointed out. Resistance to amikacin, cefotaxime and norfloxacin was observed only in strains from clinical specimens. Amikacin was the antibiotic to which 80% of multiple resistant strains showed sensitivity. The distribution of O-antigen serotypes varied by samples sources; 01, 06, 011 serotypes were the most frequently recovered. No isolates were found to be 08, 013, 014, 015 immunotype. The data indicate that serotyping can be a useful addition to antibiotic susceptibility testing for epidemiological purposes.