A prospective study of 64 patients admitted to a medical intensive care unit and 86 patients admitted to a surgical intensive care unit was done to determine the frequency of pharyngeal, intestinal, and tube site colonization with Gram-negative bacilli. Studies were carried out over a 13-week period. The pharyngeal carrier rate among the surgical patients increased by a total of 34 strains compared to 14 strains among medical patients. Similarly, the intestinal carrier rate increased by 35 strains compared to 12 strains. The increased carriage in surgical patients was related more to the presence of indwelling tubes and colonization of multiple sites in the same patient than to the use of antimicrobial drugs. Pharyngeal and rectal colonization in medical patients was related to antibiotic therapy. Indwelling tubes were used predominately in surgical patients and were a significant reservoir of these organisms.