In the present study, fry of O. mykiss fed with different levels of S. cerevisiae had a higher growth rate than those fed with normal artificial food and artificial diet covered by fish oil at the first week (C and D0). Research on the effects of strain of dietary S. cerevisiae and rearing conditions in rainbow trout revealed that supplementation of trout starter diet with S. cerevisiae may be particularly useful to increase fish growth. Moreover, differences in temperature strongly affect fish growth and metabolism (Waché et al., 2006). Noh et al. (1994) and Lara-Flores et al. (2003) demonstrated greater growth in fry fed with diets containing a probiotic supplement than those fed with the control diet without supplement. The intestinal colonization of early feeding fry with yeast may have some effect on development, for example, by accelerating the maturation of the digestive system. In older fish, dietary yeast may stimulate metabolism and growth (Gatesoupe, 2007). In the present research, the best FCR, SGR and BWG values were observed in diets D1 [artificial food mixed by 1% S. cerevisiae (w/w)] and D5 [artificial food mixed by 5% S. cerevisiae (w/w)] suggesting that these feeding diets improved feed utilization even under stress conditions. Similar results have been reported for S. cerevisiae use