A study was conducted to evaluate the effects of supplemental protease and cereal grain type on nutrient digestibility (jejunum and ileum) and performance of broilers offered diets with reduced amino acid concentrations and supplemental xylanase and phytase. A total of 624 male broiler chicks (Ross 308) were randomly distributed into 48 floor pens (13 chicks/pen" 0.07 m2/bird) and offered one of six dietary treatments with eight replicates per treatment. Dietary treatments were either maize- or wheat-based with a positive control (PC) reference diet, a negative control diet without protease (NC" 60 g/kg lower amino acid density than PC), and an NC diet with protease. The reduction in amino acid density affected (P<0.05) nutrient digestibility by varying degrees depending on cereal grain source. At 14 d of age, cereal grain and protease showed a significant interaction (P<0.05) which affected jejunal and ileal starch digestibility, whereby protease increased digestibility in birds fed wheat-based diets but not in those fed maize-based diets. Cereal grain source affected (P<0.05) nitrogen (jejunum and ileum) and digestible energy (DE" ileum), where birds fed wheat-based diets had higher digestibility than those fed maize-based diets. At 28 d of age, birds fed wheat-based diets had a higher (P<0.01) jejunal and ileal nitrogen digestibility, whereas protease reduced ileal nitrogen digestion. Protease affected ileal starch digestion in birds fed wheat, but not maize-based diets, resulting in a significant cereal grain × protease interaction (P<0.05). Wheat-based diets had a higher DE than maize-based diets in both the jejunum and ileum. From 15 to 35 d of age, cereal grain source (P<0.05) affected performance, whereby broilers offered maize-based diets had better performance than those fed wheat-based diets.