Turkey chickens were fed for 4 weeks with starter diets containing 0.1, 0.2 and 0.4% mannan-oligosaccharide (groups MOS-0.1, MOS-0.2, and MOS-0.4, respectively) and a control diet without oligosaccharides. Supplementation of diets with mannan-oligosaccharide had no statistically significant influence on growth or relative weight of the stomach, small intestine or colon. MOS had a different influence on the weight of the caeca and caecal digesta parameters. Gradually increasing mannan supplementation to the diet lowered caecal tissue and digesta weight compared with the control group. The dry matter of caecal digesta was unaffected by the added mannan, the ammonia and nitrogen contents were reduced, but the pH of the caecal content was significantly greater after mannan supplementation than in the control group. Compared with the control diet, mannan intake caused a slight increase in short-chain fatty acid (SCFA) concentrations in the caeca, especially of propionate, but the caeca SCFA pool (as a consequence of the lower ceacal content) was significantly decreased in groups MOS-0.2 and MOS-0.4. In turkeys fed mannan-diets a decrease in the bacterial glycolytic activity in the caecal digesta was observed, especially in α-glucosidase, α-galactosidase and β-galactosidase activities. A beneficial decrease in β-glucuronidase activity was observed only with the highest amount of MOS in the diet (0.4%).