An experiment was conducted to assess the optimum level of inclusion of spineless cactus (Opuntia ficus indica) on growth performance of lambs receiving straw based diets. Eighteen lambs averaging 12.4 kg body weight (3-4 months of age) were divided into 3 groups of six each. Group 1 (T1) received a control diet comprising urea treated finger millet (Eleusine coracana) straw and a compounded feed mixture (CFM) based on corn. Groups T2 and T3 received the same diet except that the spineless cactus replaced the CFM at 30 and 50 per cent, respectively. The growth trial lasted for 12 weeks. Digestion trial was completed in all animals during the last week of the study. The intake of dry matter and nutrients were similar among the treatment groups. The average daily body weight gain (g per day) was 72.1, 74.2 and 70.0 for T1, T2 and T3, respectively with no significant difference among the groups. Animals in T1 had a significantly higher (P<0.05) digestibility of dry matter and organic matter than those in T1 and T2. Lambs of group T3 had a significantly lower (P<0.05) voluntary water intake than those of T1 and T2. Although the CFM was replaced at 50 per cent by spineless cactus (group T3), the maximum potential of lambs to consume the dry matter by spineless cactus was only 40 per cent. It was concluded that the spineless cactus could be included in the diet of growing lambs replacing 40 per cent of the CFM, equivalent to 28 per cent of the total diet.