Experimental gastric ulcer was induced in Wistar rats by a Clamping-Cortisone method. Candida albicans infection was achieved by oral administration. Animals were sacrificed after the 4th and 13th week. Gastric ulcers were investigated by immuno-fluorescence using anti-Candida albicans antisera. In rats sacrificed after the 4th week, Candida albicans was fluorescent positive at the ulcer base. As in man, many fluorescent histiocytes were seen not only around, but far from the ulcerated lesions. Inflammatory cells were also numerous at the ulcer base. These findings support the hypothesis that, in man Candida albicans infection at the ulcer base is not merely a simple growth, but that it is also involved in the poor curability of gastric ulcer. In animals sacrificed at the 13th week, fibrous change with collagen formation at the ulcer base was less prominent than in those sacrificed at the same period but not infected by Candida albicans. Possibly, this is due to the fact that Candida albicans inhibits fibrous change and collagen formation (mechanisms of healing) in the gastric ulcer.