The involvement of cell membrane fatty acids in resistance of Pseudomonas aeruginosa to Quaternary Ammonium Compounds (QACs) stresses was investigated. The strain was grown in a medium with increasing concentrations of different biocides: two QACs, and two non-QACs. In the presence of two QACs only, the strain was able to grow with increasing concentrations. During cellular adaptation to QACs, the resistance to the same biocide increased. A principal component analysis was performed with whole of fatty acid compositions which highlighted a specific variation for the cultures in presence of QACs. These modifications gave evidence of the outer membrane involvement in cellular response to the presence of QACs.