Preincubation at 0 C considerably increased the bactericidal action of 0.4% nonanoic and decanoic acids on Escherichia coli K-12 154. This lethal effect seemed to be dependent on the media used to grow the bacteria. Stationaryphase cells were more sensitive than those from exponential cultures. A mutant (FA31) resistant to the bactericidal action of "cold shock" and 0.4% decanoic acid was isolated from E. coli FA23 (an E. coli 154 derivative able to grow on 0.1% decanoic acid) by a recycling selection procedure. Other E. coli strains tested showed behavior similar to that of strain K-12 154. The chilling of cells as a tool to improve the bactericidal action of fatty acids in foods is discussed. Fatty acids are used as antimicrobial food additives (1). The growth of gram-negative and-positive bacteria is inhibited by fatty acids (2, 3, 5, 8-11), the latter group being more sensitive to the fatty acid action (3, 10). The presence of a lipopolysaccharide layer, which typically surrounds the walls of gram-negative organisms, is suggested to be involved in the resistance to long-chain fatty acids (10). In Escherichia coli, the extent of growth inhibition by fatty acids is dependent on many variables, among which are the chain length of the added fatty acid and the culture medium in which the bacteria were grown (J. P. Fay and R. N. Farfas, J. Gen. Microbiol., in press). In this work we show that a cold pretreatment of cells greatly increases the bactericidal effect of some fatty acids on E. coli, and we suggest that this fact could be helpful in finding optimal conditions for the use of these compounds as preservatives for foods.