Abstract. Reliable estimates of diet to tissue fractionation of fatty acid (FA) stable carbon isotopes are essential for the development of techniques employing these biomarkers. In this work, Atlantic pollock (Pollachius virens) was used as a model species to investigate fractionation arising from metabolic processes during assimilation of dietary FA into serum and liver; we also examined fractionation occurring from mobilization of FA from liver during fasting. Pollock were fed diets containing FA of known isotopic composition, and serum and liver were collected postprandially and after fasting. Lipids were isolated from these tissues, and four polyunsaturated FA (PUFA), 18:2n-6, 18:3n-3, 20:5n-3, and 22:6n-3, within triacylglycerols (TAG), a specific lipid class associated with fat storage, were analyzed for their stable carbon isotope ratios. For 18:2n-6, 20:5n-3, and 22:6n-3, there was no discrimination between diet and serum in postprandially sampled fish, suggesting that fractionation did not occur during hydrolysis and esterification for most PUFA examined here during digestion and transfer into serum. There was a similar lack of fractionation for all four FA between fasted liver and serum, indicating that the assembly of these FA into TAG and their release into serum were not associated with fractionation. However, apparent fractionation was variable and inconsistent for all FA between diet and postprandial liver, indicating a failure of liver TAG to fully reflect the new diet. These data will allow δ 13 C values of three PUFA in postprandial serum to be incorporated into mixing models to estimate recent diet in gadoids. Further controlled feeding studies, under conditions that elicit physiological responses that are similar to those of fish in their natural environment, will be necessary before reliable estimates of longer-term diet, derived from δ 13 C in liver FA, will be possible.