This study was carried out to evaluate the effect of different levels (0.05%, 0.1%, 0.15% and 0.2%) of yeast probiotic on the growth parameters and survival rate of rainbow trout fingerlings. The study was conducted in a completely randomized design with 5 treatments and 4 replicates (consisting of 20 experimental units or ponds). The yeast probiotic was mixed with a particular commercial diet in order to obtain four experimental diets. The experimental diets were fed to fish with a mean body weight of 23 grams at the start of the experiment in 16 ponds for 50 days (excluding the ponds of the control group). The fish were evaluated at days 10, 20, 30, 40 and 50 of being fed by the experimental diets in order to obtain growth rate, feed conversion rate and survival rate. The results indicated that the yeast cells of Saccharomyces cerevisiae at the consumed levels were established in the gastrointestinal system of the fish and had positive and significant effect on  the growth parameters (final body weight, growth rate, feed conversion percentage and survival rate) (p