The phenomenon of antimicrobial agent dependence has been infrequently described, with vancomycin-dependent Enterococcus faecalis first reported in 1994 (3) and then occasionally thereafter (7, 9). Although heteroresistance of the Acinetobacter baumannii-Acinetobacter calcoaceticus complex (ABC) to colistin has been described, the dependence of the ABC on antimicrobial agents has not been reported (5). We describe the isolation of a colistin-dependent subpopulation of the ABC from a clinical isolate.