In vitro fermentations were carried out by using a model of the human colon to stimulate microbial activities of gut bacteria. The model consisted of a three-stage culture system. Bacterial populations were evaluated under the effect of three types of arabinoxylan, a nonstarch polysaccharide derived from wheat, the water-unextractable arabinoxylan fraction (WU-AX), WU-AX pretreated with exogenous xylanase and the soluble water-extractable arabinoxylan fraction (WE-AX). The xylanase pretreated (WU-AX) had a stimulatory effect upon colonic bifidobacteria throughout all three vessels. Counts of Bacteroides spp. and Clostridium spp. were also both significantly reduced. Addition of the WU-AX substrates to the first vessel resulted in induction of bacterial synthesis of extracellular hydrolytic enzymes xylanase and ferulic acid esterase which are both required for bacterial metabolism of WU-AX; this induction was significantly greater with the xylanase treated WU-AX.