As it is now possible as well as practical to identify nearly all nonfermentative Gram-negative bacilli recovered from clinical material, 565 strains of these bacteria were studied in respect to four features: incidence, distribution, potential pathogenicity, and antibiotic susceptibility. Four species, previously known to be opportunistic pathogens, Pseudomonas aeruginosa, P. maltophilia, Acinetobacter anitratus (Herellea vaginicola), and A. Iwoffi (Mima polymorpha), were the most commonly encountered. P. pseudoalcaligenes and P. fluorescens were also frequently encountered. Seven species of questionable pathogenicity were less common; these were P. stutzeri, P. acidovorans, P. putida, P. alcaligenes, P. multivorans, P. diminuta, and Chromobacterium typhiflavum. Bordetella bronchiseptica and Xanthomonas species, occasionally encountered, appeared to be nonpathogenic for man. Three of the five species of Moraxella associated with man were encountered: M. nonliquefaciens, M. osloensis, and M. phenylpyrouvica. Only one strain of Alcaligenes faecalis, of doubtful pathogenicity, was found. Some strains of A. odorans were pathogenic. Most strains of flavobacteria were not clinically significant; none of these corresponded biochemically to King's Flavobacterium meningosepticum.