The aim of this study was to investigate the combined effects of thyme essential oil and Saccharomycetes on blood counts and body productivity in rainbow trout. The fishery complex demonstrates positive dynamics in all key economic indicators. The use of feed additives in a complete fish diet is one such practical solution that not only improves nutritional efficiency, but also helps to reduce stress and control diseases by strengthening the immune system of fish. The study was carried out in

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... e fish farm of the Leningrad region, on rainbow trout. 3 groups were taken, 5 in each group. The control group received only the main diet, only Saccharomycetes were introduced to the diet of group 2, and thyme essential oil was added to the main diet of group 3 in addition to Saccharomycetes. The mixture was administered at a dose of 2% of the norm was the essential oil of thyme, and 0.1% of saccharomycetes. Physico-chemical indicators of water, including temperature, were at the same level in all groups, within the physiological norm. The experiment lasted 30 days; at the end of the experiment, blood was taken to study blood parameters. Statistical analysis was carried out using STATISTICA 6 software.The present study showed that the combination of Saccharomycetes and thyme essential oil promotes the full development of fish and the efficient use of feed. Can theoretically successfully increase growth rates, survival rates, and digestive enzymes in farmed rainbow trout. Also, their inclusion in the diet can be useful for enhancing immune parameters and antioxidant responses and, therefore, can be included in the diet of this fish species as a safe feed additive.