The susceptibilities of 159 clinical isolates of glucose nonfermentative gram-negative bacilli were determined for eight new monobactam or beta-lactam antibiotics. Imipemide (N-formimidoyl thienamycin) was effective against the largest number of species, although not against Pseudomonas maltophilia. Cefoperazone and ceftazidime, but not cefsulodin, were active against infrequently isolated Pseudomonas species. Aztreonam, moxalactam, cefotaxime, and ceftizoxime demonstrated selective activity against several species, including certain amino-glycoside-resistant isolates.