Upon exposure to 6,000 U of polymyxin B sulfate per ml, cells of the polymyxin-sensitive PAO 1 strain of Pseudomonas aeruginosa displayed in thin sections long projections arising from the outer membrane of the cell wall and extensive cytoplasmic degradation with accumulation of cytoplasmic membrane infoldings. Polymyxin-resistant isolates derived from the PAO 1 strain, however, grew well in the presence of 6,000 U of polymyxin per ml and exhibited none of these effects, having instead the appearance of a typically healthy cell. Freeze-etching of cells of the sensitive strain grown in basal medium without polymyxin revealed a concave cell wall layer studded with numerous particles. Freeze-etching of cells of the resistant isolates grown in basal medium containing 6,000 U of polymyxin per ml revealed a concave cell wall layer (i.e., the outer half of the outer membrane) in which most of these particles were absent. Thus, acquisition of resistance to polymyxin was correlated with an alteration in the architecture of the outer membrane. When the resistant isolates were grown in the basal medium lacking polymyxin and then freeze-etched, the particle distribution in the concave cell wall layer resembled that of the sensitive parent strain. The cells had regained sensitivity to polymyxin upon suspension in medium containing 6,000 U/ml as determined by their failure to grow and by internal damages seen in thin sections. These cells also had acquired increased sensitivity to ethylenediaminetetraacetate, whereas the polymyxin-resistant cells grown in the presence of polymyxin were resistant to lysis by ethylenediaminetetraacetate. The polymyxin-resistant isolates were not stable mutants but instead represented an adaptive response to the presence of polymyxin in the medium.