Compared with conventional indoor rations, consumption of grazed grass by cattle improved the fatty acid profile of beef, by increasing the polyunsaturated fatty acid (PUFA) and conjugated linoleic acid (CLA) concentrations and decreasing the n-6:n-3 PUFA ratio (French et al., 2000). Supplementation of grazing cattle with sunflower oil (S), increased the concentration of CLA in muscle but also increased the n-6: n-3 PUFA ratio (Noci et al., 2006). Supplementation with fishoil (F), a source of long-chain n-3 PUFA might decrease this ratio while enhancing CLA synthesis. However, increasing the concentration of PUFA, particularly the long-chain PUFA, may predispose muscle lipids and pigments to oxidation, with consequent impairment of lipid and colour stability. The objectives were to determine the effect of supplementation of grazing cattle with S and F on (i) tissue n-3 PUFA and CLA concentrations and (ii) the colour and lipid stability of beef.