Because the integrity of cellular membranes is critical to insects exposed to potentially lethal low temperatures, there is tremendous advantage conferred to insects that can adjust the composition of membrane fatty acids (FA’s) in response to low temperature. The flesh fly, Sarcophaga crassipalpis was subjected to 4^oC for various time periods and the composition of membrane FA’s were compared to flies that were not subjected to cold treatment. Membrane composition was altered in response to low temperature: the most conspicuous change was an increase in the length of fatty acid chains, a condition that theoretically decreases membrane fluidity. However, the greatest individual fatty acid increase was seen for 5,8,11,14 eicosadienoic acid, a highly-unsaturated fatty acid that generally increases membrane fluidity because of its high degree of unsaturation. In addition, trace amounts of a cyclopropaneoctadecanoic acid, a fluidity-enhancing cyclic fatty acid, were only seen in flies that were subjected to chilling. From microarray analysis of mRNA isolated from chilled flies, we also identified several membrane-related genes that increase expression as a result of chilling. Among these are phospholipase D, enoyl CoA-hydratase, paxillin, sema-1a, and GAP-1.