The membrane pacemaker theory of ageing proposes that the polyunsaturated fatty acid (PUFA) composition of membrane lipids of a species is an important determinant of its maximum lifespan. We report three experiments using the blowfly Calliphora stygia, where this theory was tested by manipulation of dietary fat profile. Although the fat profile of the larval diet resulted in small alterations of individual membrane fatty acids, it had no effect on the peroxidation index (PI) of membrane lipids and furthermore had no effect on maximum lifespan. Similarly, manipulation of the fat profile of the adult diet resulted in small changes in individual fatty acids, but had no effect on the PI of membrane lipids. There was a small increase in maximum lifespan when adult diet was supplemented with PUFA in form of vegetable oils but no effect when diet was supplemented with pure PUFA. This difference is possibly due to antioxidant content of vegetable oils. The relative refractoriness of membrane PI to dramatic changes in response to diet is similar to the situation in the rat. These results also indicate the blowfly is unable to convert 18-carbon PUFA to more highly polyunsaturated 20- and 22-carbon PUFA.