Yogurt containing Lactobacillus gasseri OLL2716 (LG21 yogurt) is reported to improve Helicobacter pylori-induced gastric mucosal inflammation in clinical studies. However, other beneficial effects of LG21 yogurt have not been clarified. Therefore, we examined whether LG21 yogurt exhibits a gastroprotective action against acute gastric lesion or antral ulcer in rats. Moreover, the mechanism of gastroprotective action was also evaluated. After fasting, acute gastric lesions were induced by 0.6 M HCl. Gastric mucosal folds were stained by oral administration of methyl violet. Antral ulcers were induced by the combined administration of diethyldithiocarbamate and HCl in refed rats after fasting. LG21 yogurt was orally administered before HCl treatment or staining the mucosal folds. LG21 yogurt significantly and dose-dependently inhibited the formation of acute gastric lesions, and this gastroprotective action was attenuated by pretreatment with indomethacin. LG21 yogurt also significantly increased prostaglandin E₂ generation in the gastric mucosa. Stained length of gastric mucosal fold was reduced by LG21 yogurt. Antral ulcer formation was also significantly inhibited by LG21 yogurt. From the above results, it was found that the ingestion of LG21 yogurt is useful for the prevention of gastric ulcer. Moreover, endogenous prostaglandin was suggested to be one of the gastroprotective mechanisms of LG21 yogurt.