S. boulardii have been used to prevent or to treat intestinal infection. The aim of this study was to analyse the proteins involved in simulated gastric environment resistance. Different yeast mutants were grown in YPD medium, washed with YP twice and incubated at 37°C for 1 h in a simulated gastric environment or in different constituents of gastric environment. Our results evidence the highest tolerance of the probiotic S. boulardii and that NaCl or pepsin do not affects cells viability significantly. Furthermore, we could observe that NaCl addition protected cells from low pH deleterious effects. Vanadate addition to cells exposed to low pH plus 80 mM NaCl decreases cells resistance suggesting a partial role of P-Type ATPases in the stress response. Measurements of intracellular pH in different mutants exposed to low pH and NaCl fased on the distribution of 14-C propionic acid suggest that intracellular pH is not related with viability. Since sodium ions confer protection effect during acidic stress we measure Na accumulation and efflux, by atomic emission spectrometry, and expression of genes involved in Na homeostasis. Ours results suggest that NHA1p is required for sodium tolerance at acidic pH and that expression of ENA 1-4 and NHA1 genes have a complementary action to maintain the intracellular steady-state concentration of Na. The H⁺-ATPase specific activity is higher in S. boulardii than in W303 cells. We also compared the activity of plasma membrane H⁺-ATPase in S. cerevisiae cells grown in media with an initial pH of 2.0 -6.0. The highest activity was observed at higher pH, suggesting a role for this protein in low pH resistance. Measurements of membrane potencial are in course. The goal is to examine the role of membrane potencial in the yeast growth at low pH.