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The effects of the pore-forming domain of colicin A on acyl chain order and lipid organization of model membrane system derived from Escherichia coli were investigated using nuclear magnetic resonance (NMR) methods. A fatty acid auxotrophic mutant of E. coli was grown in the presence of deuterium-labeled oleic acid and was used for the NMR study. The colicin A pore-forming domain was inserted in the membrane and catalyzed at acidic pH. A major perturbing effect on acyl chain order was observed upon insertion of the pore-forming colicins but bilayer organization remained unaffected and exhibited stability.