Eleven antibiotics were tested against 1,194 Gram negative bacilli isolated from urinary tract infections at the National University Hospital Center at Cotonou. Among the betalactams tested, only cefotaxime remained active against most of the bacteria tested: 90% of the strains of Escherichia coli and 75% of the strains of Enterobacter cloacae were sensitive. Ampicilline, on the other hand, had lost its activity even on strains which are usually the most susceptible. Thirteen percent of the E. coli strains were sensitive. This reduction in antibiotic activity against bacterial strains in Cotonou, which concerned to various degrees the tetracyclines, chloramphenicol, cotrimoxazole, is less pronounced for the amino-sides (gentamicine and netilmicine), and the quinolones of which nalidixique acid was active against 83.9% of the strains of E. coli. The low frequency of isolation of wild type strains (sensitive to betalactams) is probably the consequence of strong selection pressure due to a massive, and uncontrolled use of antibiotics in Cotonou.