Moraxella and Acinetobacter are two quite different bacterial genera whose major similarity is that representatives of each may look like each other on Gram stain. Approximately 1 to 3% of all nonf astidious Gram-negative bacilli isolated in the average hospital laboratory are Acinetobacter species. Acinetobacter species are free-living organisms which commonly are present in soil and water. Most laboratory isolates of Acinetobacter are unassociated with significant infection. Many studies of the antibiotic susceptibility of Acinetobacter species have been published but few have employed quantitative methods. Several investigators have shown minocycline to be more active than tetracycline against Acinetobacter isolates. Moraxella is less often recognized in the clinical laboratory than Acinetobacter. Although colonization in the absence of disease and culture contamination with Moraxella are common, clinical isolates are more likely to be involved with infection than those of Acinetobacter. The prevention of Acinetobacter infections will depend, in large part, upon the effectiveness of hospital infection-control programs.