The aim was to investigate how blueberry husks and/or mixtures of probiotic strains (Lactobacillus crispatus DSM16743, L. gasseri DSM16737 and L. plantarum DSM15313 (LABmix), or Bifidobacterium infantis DSM15159 and DSM15161 (BIFmix)) affect colonic fermentation, caecal counts of lactobacilli, bifidobacteria and Enterobacteriaceae, body weight gain, and blood concentrations of carboxylic acids (CA) and ammonia in rats. Dietary fibres in blueberry husks were fermented to 61 % in colon, and the elevated faecal excretion of fibre and protein contributed to the high faecal bulking capacity (1·3). The caecal pool of CA was higher in rats fed blueberry husks than the fibre-free control (P < 0·05), and the propionic acid proportion was higher in the distal colon than in the control group (P < 0·05). Probiotics lowered the caecal amount of CA when added to blueberry husks (P < 0·001), while the propionic acid proportion was higher with LABmix (P < 0·01) than blueberry husks only. The propionic acid and butyric acid concentrations in blood were higher in rats fed blueberry husks and probiotics than those fed blueberry husks only (P < 0·01), implying that the absorption of these acids was facilitated by the bacteria. The caecal counts of lactobacilli, bifidobacteria and Enterobacteriaceae were lower in rats fed blueberry husks than the control diet (P < 0·05). The body weight gain was partly influenced by the caecal tissue and contents weights, and BIFmix decreased the ammonia concentration in blood (P < 0·05). We conclude that colonic fermentation is differentially affected by dietary fibre and probiotics, which may be of importance when developing foods with certain health effects.