Some studies with beef cattle have shown that breed and diet affect tissue fatty acid composition and meat quality (Choi et al., 2000; Scollan et al., 2001). However, the effects of breed are often confounded with differences in growth rate and body composition. Diet also affects fatty acid composition, however, feed composition studies are often confounded by the use of mixed diets and few have compared all-forage with all-concentrate diets. This study, therefore, was designed to compare Aberdeen Angus and Holstein-Friesian breeds growing at similar rates and fed either all-forage or a high concentrate diet.