Abstract: The gastrointestinal tract is a complex digestive system, and each section has its unique physiological characteristics. Hydrogen ion concentration (pH) is an important physiological indicator of the digestive tract. It involves the process of digestion and absorption of nutrients and the growth of intestinal microbes. This study investigated the change of pH level of the intestinal tract in grass carp (Ctenopharyngodon idellus) during food digestion period. The pH levels of intestinal chyme, fluid and mucosa of the fish were measured at different time points after food ingestion. The results indicated food digestion resulted in significant decreases in pH of the intestinal tract. The pH levels of the intes-tinal chyme and fluid ranged from 6.86±0.24 to 8.43±0.10 and from 7.14±0.22 to 8.63±0.02, respectively. Within each intestinal segment, there was no significant difference in pH between the intestinal chyme and fluid at each time point. By contrast, the pH levels of intestinal mucosa were between 6.23±0.04 and 6.7±0.13, significantly lower than mea-surements made in the intestinal chyme and fluid (P<0.05), with the exception of the 12h post-ingestion. Further analyses found that the pH level of the fish diet was significantly different with the pH levels of intestinal chyme, fluid and mucosa (P<0.05). The present study advances the understanding of the digestive physiology in grass carp.