Previous work has shown that (1->4)-beta-D-endoxylanase-mediated hydrolysis of wheat bran leads to solubilization of 50% of arabinoxylans. However, xylanase efficiency on the individual bran tissues is unequal because of histological and chemical heterogeneity. We describe here the results of an immunocytochemical study that is aimed at an understanding of in situ enzyme action on bran xylans at different hydrolysis kinetic stages. Two polyclonal antibodies were used, one against xylanase and the other against (1->4)-beta-D-unsubstituted xylopyranosyl residues, to target poorly substituted arabinoxylans. These antibodies were used on optical microscopy or transmission electron microscopy sections of xylanase-treated and water-treated wheat bran. After 30 min of xylanase treatment, xylanase distribution was found to be confined to the aleurone cell walls close to the endosperm, and arabinoxylan labeling had been lost. After 75 min, xylanase had progressed throughout the aleurone and had begun to attack the nucellar layer. After 24 h, the aleurone was completely lost, while some remnants of the nucellar layer were still observable. In contrast, the morphology of the pericarp and the testa was unaltered, and no xylanase labeling in these tissues was detected. Xylanase localization was correlated to the level of arabinoxylan substitution. That way, we showed that xylanase is preferentially localized and degrades poorly substituted arabinoxylans, as shown by visible subcellular heterogeneity of aleurone and nucellus walls.