This article describes studies on the impact of effective microorganisms of the “Baikal EM-1” probiotic on the biotransformation of flaxseed meal, its organoleptic and chemical parameters, and changes in amino acid and fatty acid compositions. It has been established that after fermentation in flaxseed meal, the amount of fiber decreases, it saturates with protein, and the amount of essential and non-essential amino acids increases. We made an analysis of the feed value of biotransformed meal depending on the probiotic concentration for the purpose of further use in the feeding of carps – the most effective probiotic concentration for biotransformation was selected. The effect of replacing part of the feed with biotransformed flaxseed meal on the change in liveweight and biochemical parameters of fish blood was studied. Positive dynamics of fishes’ liveweight was noted. The inexpediency of feeding unfermented meal to carps is proved.