This study evaluates the effects of breed and sex, together with those of birth weight and litter size, on muscle fibre type characteristics in Texel (TEX) and Scottish Blackface sheep (SBF). The M.longissimus thoracis et lumborum (LTL) of TEX had a significantly higher total muscle cross-sectional area (16%), a higher total fibre number (20%) and a higher muscle CT density (5%) than the SBF but had a similar average muscle fibre size. The frequency of slow fibres in the LTL in TEX was lower than in SBF (7.5% vs. 9.6%). Muscle fibre histochemistry similarly demonstrated that the oxidative fibre frequency in TEX was 10% lower than in SBF. The inter-fibre lipid content in TEX was also significantly lower than in SBF. Correspondingly, TEX displayed higher frequency (91.7% vs. 90.9% in SBF) and higher relative total area (92.5% vs. 90.4% in SBF) of fast fibres. These breed differences in muscle fibre traits indicate underlying genetic variation, and future analyses will evaluate the link of these traits to meat quality and assess the usefulness of these traits in breeding programmes.