The prevalence of Pseudomonas aeruginosa in intensive care unit (ICU) is 19 per cent although its incidence in blood culture is only 3.5 per cent and remains the same even though the total prevalence of this bacterium increases. A review of the clinical results shows that this germ causes severe complications in only 3 per cent of patients. A study by epidemiological markers reveals the presence of 16 different IATS serotypes in the ICU, this distribution of the serotypes being similar to this observed in ambulatory patients. The serotype 12 of Ps. aeruginosa is multiresistant. In 1984, it represented 22 per cent of the isolated Ps. aeruginosa, its incidence decreases in 1985 and in 1986 (14.5%). Ps. aeruginosa seems weakly pathogenic for the surgical patients in the ICU. On the other hand, the incidence of Klebsiella pneumoniae, Serratia marcescens (014 imm. serotype resistant to tobramycin), Enterobacter aerogenes and Enterobacter cloacae increases in blood cultures proportionally to their total prevalence in ICU. These bacteria are probably responsible for hospital epidemics.