This paper studies the influence of three probiotic preparations of various microbiological composition: Bacillus subtilis (O1); Bacillus subtilis + Bacillus amyloliquefaciens (O2); Lactobacil-lus acidophilus (O3) on the growth process and histological structure of the organs of the gastroin-testinal tract of juvenile Oncorhynchus mykiss by morphometric parameters. The effect of the pro-biotic preparations led to the increase in fish growth and influenced different sections of the gastro-intestinal tract. The biggest change was found in the mid intestine and the reliable difference com-pared to the control was obtained at the following parameters: Lamina propria width, intraepithe-lial lymphocytes number of prismatic epithelium and goblet cells area. The changes in the pyloric appendages were less obvious but reported as playing an important functional role. The liver pre-served normal functional structure in all series of the experiment except for the probiotic group Lactobacillus acidophilus, where hepatocyte small-drop vacuolization was observed. That might be connected with the change of the digest activity resulting from a decrease in secretory activity of the intestinal exocrinocytes. The use of all the probiotic preparations led to the similar change in morphometric parameters, in all the groups it was possible to decrease the immune response.