Saccharomyces boulardii is a nonpathogenic yeast with proven health benefits, some of them depending on its viability. However, the living yeast is sensitive to environmental conditions and its viability is less than 1% in the faeces after oral administration. Therefore, we assessed the survival conditions of S. boulardii in aqueous suspension and in its freeze-dried form and we formulated microspheres with the former and tablets with the latter in order to preserve the viability of the probiotic. While the viability of the yeast in aqueous suspension could be maintained for one year at -20 degrees C and +5 degrees C, increasing the temperature led to almost total mortality within 14 d at +40 degrees C and 4 d at +60 degrees C. The viability of the freeze-dried yeast was preserved for one year at +25 degrees C without moisture. With 75% relative humidity, the mortality was significant at 28 d at +25 degrees C and almost total within 1 d at +60 degrees C. In vitro, whereas less than 1% of non-encapsulated or non-tabletted S. boulardii survived after 120 min at pH 1.1, both formulations in microspheres and direct compression enabled to protect the yeast from degradation in HCl and to release it viable at pH 6.8. However, despite a similar release profile from both dosage forms, the compression led to a significant decrease in the viability of the freeze-dried yeast. In conclusion, although both formulations are efficient in protecting S. boulardii in acidic condition, microspheres provide a higher entrapment efficiency and a faster release of the viable probiotic in intestinal condition than matrix tablets.