Abstract. The objective of this study was to determine the effects of breed and a diet containing linseed on the growth and carcass composition characteristics of heifers. A total of 48 Limousin (LI) and Charolais (CH) heifers with an average weight of 270 kg were assigned to two diets containing either extruded linseed (LIN) or no supplemental oilseed (CON). The target slaughter weight was set at 500 kg. The diet had no effect on any of the observed production traits. The CH heifers had higher live weight gains (P<0.001) and a lower feed conversion ratio (P<0.001). The LI heifers had a higher dressing percentage (P<0.001), higher meat to bone ratio (P<0.001), greater m. longissimus lumborum et thoracis area (P<0.05), and produced more internal and carcass fat (P<0.05). It was concluded that purebred LI heifers grew less rapidly and less efficiently but produced heavier carcasses with a more favourable meat to bone ratio compared to purebred CH heifers.