Aquatic Feed Science. The influences of xylanase added in wheat basal diet on intestine chyme viscosity and the development of villi and microvilli of Tilapia nilotica. Wheat is important cereal crop in our country. Recently, because of the rise of science and technology, the output of wheat have increased significantly and exceeded in phase and structure. Meanwhile, the flourishing developments of aquatic feed industries have been restricted seriously by the defect of raw material. Using wheat as raw material of feed, carrying out transformation and consumption by aquaculture, all of these now possess the resources feasibility. The use of wheat in aquatic feed industry as major energy feed has been restricted because of xylan. In the past few years, the application of feed xylanase with wheat basal diet has resolved the problems of the anti-nutrition of xylan successfully. Xylanase could improve the growth of aquatic animals fed with wheat basal diet. But there weren’t any researches about the mechanism of xylanase promoting aquatic animals’growth at home and abroad. The purpose of adding xylanase to wheat basal diet in this paper was to study the influences of xylanase on intestine chyme viscosity, on the developments of intestine villi and microvilli. In this study we also probed into the relationship between intestinal environment and the developments of villi and microvilli, displayed the mechanism of xylanase relieving the anti-nutrition of xylan and promoting Tilapia nilotica’s growth. Wheat basal diet was control. The tested diets were wheat basal diet added with different levels of xylanase (0.05%, 0.10%, and 0.15% respectively). Each treatment was devised with 5 repeats and each repeat had 40 male Tilapia nilotica. The fish in floating cages were fed to satiation for 75 days. The results indicated that adding xylanase to wheat basal diet could decrease intestine chyme viscosity, promote the developments of villi and microvilli of Tilapia nilotica. The intestine chyme viscosity of 0.05%,0.10%,0.15% xylanase groups ［(13.75±0.45) cps, (12.23±0.16) cps and (11.92±0.22) cps respectively］ was significantly lower than that of control ［(15.21±0.29) cps］ (P<0.01). The number and diameter of adhering granule on the villi surface of fore-gut, mid-gut and hind-gut of tested groups significantly decreased with supplement of xylanase. The height and density of intestine villi of the tested groups were significantly improved. The height, width and density of intestine villi of 0.10% xylanas group were significantly higher than that of control (P<0.01). According to the states of intestine chyme viscosity, villi and microvilli development in this study, we concluded that fore-gut and mid-gut of Tilapia nilotica were the major places of digestion and absorption. The appropriate amount of xylanase added to wheat basal diet was 0.10%. This study firstly investigated the relationship between xylanase and intestine chyme viscosity, as well as the relationship between intestine chyme viscosity and the development of villi and microvilli. From histology aspect, this study also clarified the mechanism of xylanase improving Tilapia nilotica’s growth, and put forward the concept of “fold villi”.17 Refs.In Chinese. Key words: wheat basal diet; xylanase; Tilapia nilotica; chyme viscosity; intestine villi NIE Guo-xing (College of Life Sciences, Henan Normal University, Xinxiang 453007, China), WANG Jun-li, ZHU Ming-wei, ZHOU Hong-qi//J Fish China,2007,31(1).54-61