Acquired resistance of Pseudomonas aeruginosa to beta-lactam antibiotics, e.g. ticarcillin, has the following characters: The incidence of ticarcillin-resistant strains is about 21% but it varies with hospitals from 10.9 to 35.1%. Resistance is predominantly due to beta-lactamases, which are produced by two-thirds of the strains (64.4% in 1982, 67.9% in 1983). The presence of beta-lactamase positive strains is dependent upon the type of patient, the hospital unit (e.g. urology or burns) and the nature of the specimen collected. Most beta-lactamase positive strains spontaneously produce one single penicillinase of the CARB (53.8%), OXA (30.5%) or TEM (8.7%) type. New types of beta-lactamases may develop and hydrolyse beta-lactam antibiotics such as ticarcillin, azlocillin, cefoperazone or cefsulodin. With beta-lactam antibiotics (cefotaxime, moxalactam, ceftazidime) that resist hydrolysis, bacterial resistance is due to production of a constitutive beta-lactamase of the cephalosporinase type. This enzyme has recently appeared and its incidence is low (6.8%). Associated beta-lactamases (e.g. penicillinase plus cephalosporinase) are exceptional.