Onyango, E. M. and Adeola, O. 2011. Dietary cholecalciferol lowers the maximal activity of intestinal mucosa phytase in ducklings fed low-phosphorus diets. Can. J. Anim. Sci. 91: 399-404. The role of cholecalciferol and phosphorus in the regulation of intestinal mucosa phytase was investigated in White Pekin ducklings. Ninety-six 7-d-old male ducklings were grouped by weight into six blocks of four cages with four ducklings per cage. The average weights of the individual birds in the six subgroups were: 282, 270, 263, 255, 247 and 236 g, respectively. Four corn-soybean meal-based mash diets were randomly assigned to cages within each block. The four diets consisted of cholecalciferol at 0 or 75 µg kg^-1 and total phosphorus at 3.6 or 7.0 (or 0.12 or 0.46 available phosphorus, respectively) g kg^-1 in a 2×2 factorial arrangement. Birds were fed the experimental diets for 9 d under conditions that excluded ultraviolet light. Ducklings fed diets with cholecalciferol concentration at 75 µg kg^-1 of diet showed a lower maximal activity and K_m value of intestinal mucosa phytase (P<0.05), and higher apparent ileal digestibility of calcium (P<0.0001). Ducklings fed diets with total phosphorus concentration at 7.0 g kg^-1 of diet showed no change in intestinal mucosa phytase activity, but had higher weight gain, feed intake, tibia ash, ileal digestibility of phosphorus (P<0.0001), and feed efficiency (P<0.001). In conclusion, dietary cholecalciferol decreased the maximal activity and K_m value of intestinal mucosa phytase in ducklings fed low-phosphorus diets.