Hundred fifty rainbow trout of an average weight of 108±6.79g have been kept for sixty days in 700L tanks and various temperature regimes. They were fed two different diets, the one with probiotics containing Pediococcus acidilactici (CNCM MA 18/5M) and the other one without probiotics. All groups gained weight, however observed growth rate and weight gain have been considerably lower when fish over passed the 200g size class. No significant differences were shown between the two groups kept in lower temperatures in terms of growth rate, weight gain, feed conversion and protein utilization. Trout kept in natural temperature conditions fed either with probiotics or without probiotics, did not differ dramatically in between them in terms of growth rate and weight gain. However the ones fed with probiotics have experienced better feed conversion and protein utilization. Fish which have received dietary probiotics had lower serum cortisol levels and better Na+ and Cl- serum equilibrium than those which have not received probiotics, indicating that probiotics increased the stress resistance of experimental fish, even after the oxygen depletion test. Results of this study confirm that P. acidilactici in rainbow trout enhances its welfare and disease resistance and therefore should be used even at high size classes (100 -200g).