Wheat-based diets with or without wheat distiller's dried grains with solubles (DDGS) were tested with or without supplementary xylanase (4000 U kg-1 feed) in a 2 2 factorial arrangement. In eight ileal-cannulated barrows, xylanase improved the apparent ileal digestibility of energy and threonine in wheat (P < 0.05), but not in wheat DDGS diets (P > 0.10). Xylanase did not affect total tract digestibility of energy or P (P > 0.10). In 72 grower-finisher pigs, xylanase did not increase growth performance of pigs fed either wheat or wheat DDGS (P > 0.10). The differential response of supplementary xylanase to wheat versus wheat DDGS diets indicates that the arabinoxylans in wheat DDGS did not match the specific xylanase activity.