Oral preventive treatment of gnotobiotic mice by Saccharomyces boulardii significantly decreased mortality following Clostridium difficile infection. A single S. boulardii ingestion protected 16% of mice, whereas 56% were protected when S. boulardii was given continuously in the drinking water. No direct antagonistic effect of the yeast on C. difficile numbers was detected, whereas a modulation of fecal cytotoxin production was demonstrated.