The fatty acid composition and temperature/growth characteristics of a psychrophilic bacterium, strain ACAM 456, isolated from Antarctic sea-ice is reported. The bacterium produced acyl components that may be grouped in three different carbon chain types: even-chain, odd-chain and iso-branched odd-chain. The proportions of these chain types varied according to growth temperature, and were manipulated by growth on L-serine, t-leucine or propionic acid as sole carbon sources. De novo fatty acid synthesis was investigated using sodium [1-¹⁴C]acetate, L-[U-¹⁴C]leucine and L-[U-¹⁴C]serine as radioactive precursors. Compared with a control culture, resuspension of midexponential phase cells in artificial seawater led to a change in the selection and/or intracellular availability of acyl chain primer molecules. The proportion of radiolabel incorporated into even-chain length components from cells declined, whereas the percentage of radiolabel present in odd-chain length components increased. An increase in incubation temperature augmented this effect, and also elicited a rise in the proportion of label present in branchedchain products. ACAM 456 manipulated the utilization of acyl chain primer molecules as an adaptive response to changes in environmental conditions. In particular, the regulation of odd-chain length fatty acids is described as a novel adaptational response.