The objective of this study was to evaluate the substitution of soybean meal for castor seed meal (CSM) in diets for feedlot lambs and the effects of these diets on their ingestive behavior. Fifty male Santa Inês lambs were used. The diets were composed of Tifton 85 hay and a concentrate containing detoxified CSM substituting 0, 25, 50, 75, or 100 % of the soybean meal. There was no effect (P > 0.05) of the CSM levels on the feeding, rumination, idle times, chews and time spent chewing per bolus, total chewing time, number of boli chewed, and number of chews per day. The dry matter (DM) intake decreased linearly (P < 0.05), but did not affect the neutral detergent fiber (NDF) intake. The feeding and rumination efficiencies had a quadratic response (P < 0.05). The experimental diets did not affect (P > 0.05) the numbers of feeding, rumination, and idle periods, but had a quadratic effect (P < 0.05) on the time per feeding activity and on the chewing periods. Substitution of soybean meal for detoxified CSM reduces the DM intake but does not change the ingestive behavior.