In June, 1993, an Enterobacter aerogenes strain was isolated in the Hospital de la Creu Roja from Hospitalet de Llobregat (Barcelona), which was resistant against all beta-lactams, including imipenem. Since is unusual in Enterobacter sp. to isolate imipenem resistant strains, we decided to study its resistance mechanism.To study the E. aerogenes 174004/H resistance mechanism beta-lactamase isoelectrofocalization was performed together with the determination of kinetic constants in order to characterize the beta-lactamase, and a polyacrylamide gel electrophoresis in order to observe the profile.The strain of E. aerogenes 174004/H exhibits a chromosomic beta-lactamase with a pI higher than 9.2 and a decrease in an outer membrane protein of 42 kDa, probably a porine.E. aerogenes 174004/H resistance against imipenem is due to an hyperproduction of a chromosomic beta-lactamase with a pI higher than 9.2 and to a 42 kDa decrease of an outer membrane protein expression.