One strain of Acinetobacter calcoaceticus biovar anitratus caused colonization of 111 patients admitted to an intensive care unit (ICU) during a 2-year period. All patients were intubated and had received antibiotic therapy prior to colonization. Morbidity due to the organism was about 1%. The colonization rate showed a decreasing trend during the study period, but no seasonal variation. The strain was found in the air in a low concentration and on the hands of 8-13% of the members of the staff. No chronic carriers were found.