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The aim of this study was to evaluate the effect of age and sex on histochemical and morphometric characteristics of muscle fibres (myocytes) in lambs born by single or twin birth. Forty lambs were fed mother’s milk and slaughtered at 60 days; 40 weaned at 60 days and fed until 120 days with flakes (60%) and hay (40%) and then slaughtered. Muscle tissues were obtained from two muscles, namely m. psoas major and caput longum m. tricipitis brachii of all lambs. The muscles were stained for myosin ATPase, and succinic dehydrogenase. For each fibre type, area, perimeter, maximum and minimum diameter were measured and slow-twitch oxidative fibres, fast-twitch glycolytic fibres, fast-twitch oxidative-glycolytic fibres were histochemically differentiated. An image-analysing system was used. At 60 days, females had fibres larger than males while the opposite was observed at 120 days. Moreover, at 60 days, the lambs born by single birth had fibres larger than those born by twin birth, while the opposite was observed at 120 days. The fast-twitch glycolytic fibres had the largest size, followed by the slow-twitch oxidative and the fast-twitch oxidative glycolytic fibres. The dimensions of fibre types in caput longum tricipitis brachii muscle were larger than in the psoas major muscle (P < 0.001). This study provides important data on basic characteristics of lamb muscles that, as intrinsic factors, may influence some aspects of meat production and quality.