A 60-day experiment was carried out to study the e¡ects of vitamin C [ascorbic acid (AA)] on the growth, digestive enzyme activities and intestinal microbial population. Diets with six levels (0.0, 21.4, 45.1, 69.5, 93.6 and 142.1mg AA kg À 1 diet) of supplemental ascorbyl polyphosphate were fed to juvenile Jian carp (Cyprinus carpio var. Jian) (12.63 AE 0.02 g). The speci¢c growth rate (SGR), feed e⁄ciency and productive protein value were signi¢cantly improved with increasing AA levels up to 45.1mg AA kg À 1 diet (Po0.05). The hepatopancreas and muscle AA concentrations were increased with increasing dietary AA levels up to 69.5 and 45.1mg kg À 1 diet respectively (Po0.05). The activities of intestinal trypsin, chymotrypsin, lipase, a-amylase, Na 1 , K 1 -ATPase, alkaline phosphatase, gamma-glutamyl transpeptidase and creatinkinase were all positively a¡ected by the AA supplementation (Po0.05). Intestinal Lactobacillus and Bacillus were increased with increasing AA levels up to 45.1mg AA kg À 1 diet (Po0.05), while intestinal Escherichia coli decreased with increasing AA levels up to 45.1mg AA kg À 1 diet (Po0.05). In conclusion, AA improve the digestive capacity and intestinal microbial population of Jian carp, and the dietaryAA requirement for SGR of juvenile Jian carp was 40.9 mg AA kg À 1 diet.