Four castrated crossbred (Karagouniki♂ × Friesian♀) rams were used in a 4 × 4 Latin-square design to determine the apparent digestibility of four diets based on alfalfa hay and maize grain and containing varying proportions of whole cottonseed (WCS). The proportions of WCS studied were 0, 175, 355 and 530 g/kg diet dry matter (DM) and the respective apparent digestibility coefficients of DM were 0·744, 0·716, 0·689 and 0·625; of organic matter, 0·772, 0·739, 0·718 and 0·643; and of neutral-detergent fibre (NDF), 0·409, 0·481, 0·522 and 0·472. The gross energy (GE) apparent digestibility coefficients were 0·745, 0·722, 0·704 and 0·641, for the same order of treatments. The effect of the incorporation of WCS instead of maize grain in the diet in significantly reducing DM, OM and GE digestibility was evident. The lowest digestible energy value of WCS was observed in the treatment with the highest level of WCS.