Purified non-starch polysaccharides (NSP) affect intestinal nutrient flow and hence microbial fermentation in pigs. Therefore, we investigated the effect of NSP differing in viscosity and fermentability on colonic fermentation and bacterial populations in eight ileal-cannulated barrows (BW 30 kg) fed a cornstarch-casein based diet supplemented with 5% low fermentable, low viscous cellulose (CEL), low fermentable, high viscous carboxymethylcellulose (CMC), high fermentable, low viscous oat beta-glucan (LG), or high fermentable, high viscous oat beta-glucan (HG) in a double 4 x 4 Latin square. Ileal DM flow was higher (P<0.05) for CEL, LG and HG than for CMC. Thus, more fermentable substrate entered the colon resulting in higher (P<0.01) SCFA levels in faeces with CEL, LG and HG. The LG raised (P<0.05) faecal molar proportion of butyrate 1.5 to 3 folds compared to CEL and CMC. LG also increased branched-chain fatty acids compared to CMC, indicating increased colonic fermentation of branched-chain AA that coincided with increased (P<0.05) flow of protein into the large intestine. In contrast, faecal counts of C. perfringens cluster were lower (P<0.05) for LG and CMC compared to CEL and HG. Less colonic fermentable substrate with CMC corresponded to lower faecal numbers of lactobacilli and higher numbers of Bacteroides-like and Enterobacteriaceae (P<0.05). In conclusion, effects on microbial fermentation were more linked to the chemical structure of individual NSP than to their shared physical properties, such as viscosity and fermentability. (C) 2010 Elsevier B.V. All rights reserved.