The objective was to determine the effects of fermentable carbohydrates on phosphorus (P) metabolism, the chemical composition of the faecal mixed bacterial mass (MBM) and the microbial activity in the large intestine (LI) of pigs. Eight barrows (mean BW 35.9 +/- 0.9 kg), fitted with simple T-cannulas at the terminal ileum, were either fed a low-P corn-soybean meal-based control diet or 75% of the control diet supplemented with 25% cellulose, starch or pectin according to a 4 x 3 Latin Square design. Both pectin and cellulose caused higher faecal than ileal P recoveries. Ileal volatile fatty acids (VFA) levels were more pronounced for the starch (p < 0.05) rather than the cellulose and pectin treatments, whereas pectin resulted in a higher faecal VFA concentration in comparison to starch and cellulose (p < 0.05). The differences in faecal VFA concentrations corresponded to the pH values obtained in faeces. The N content of MBM was higher (p < 0.05) when cellulose was supplemented. Pectin caused a decrease in the P content of the MBM compared to the control (p < 0.05). As a result, the N:P ratio was significantly higher for the pectin (N:P = 4.33) than for the control treatment (N:P = 2.63), while the Ca:P ratio remained constant for all treatments, suggesting changes in the accumulation of N, P and Ca in MBM, probably due to changes in the species composition and activity of the microflora.