The effects of curdlan and gellan gum on the gastrointestinal function were studied, and the morphological structure of the intestinal mucosal surface was observed by scanning electron microscopy of rats fed curdlan and gellan gum diets for four weeks. The rats fed the curdlan diet showed a significant increase in the weight of the cecum and its contents and a decrease in fecal weight as compared to the rats fed a cellulose diet. On the other hand, the rats fed the gellan gum diet showed a weight loss in cecal contents and weight gain in colonic contents. The transit time of the gastrointestinal tract was extended by curdlan supplementation whereas it was shortened by gellan gum supplementation. The surface structures of the ileal and cecal mucosa were markedly abnormal in the rats fed the curdlan diet: the microvilli were tightly packed and had fallen out at places. In the gellan gum-fed rats, the tops of the ileal and cecal microvilli adhered to one another and were covered with their contents. There was no difference in the surface structure of colonic mucosa among the cellulose, curdlan and gellan gum diet groups.