The spread in Europe of a single multiresistant strain of Pseudomonas aeruginosa serotype O12 has been suggested. This bacterium was responsible for a nosocomial outbreak in our hospital in 1988-1989. Three different epidemiological methods were used to analyze 30 strains isolated during five consecutive years. Protein profile analysis and chromosomal DNA fingerprinting with four different enzymes revealed closely related patterns. rRNA gene restriction fragment length analysis performed with a digoxigenin-labelled probe showed identical hybridization patterns with four to six bands according to the endonuclease used. Combination of the three typing methods showed genotypic homogeneity of these Pseudomonas aeruginosa O12 strains, despite a relative increase in their antibiotic resistance.