The aim of the study was to assess the effect of phytogens of essential oil crops – fennel (Foenículum vulgáre) and coriander seed (Coriándrum sátivum) on the formation of microbiota in calves in the dairy period. The studies were carried out on calves of the dairy period of the black-and-white breed, on the basis of the laboratory of immunobiotechnology and microbiology of VNIIFBIP animals in 2023. The determination of the microbiome of the scar content was carried out by T-RFLP analysis. In the course of the conducted studies, there was a tendency to increase the live weight of calves of the experimental groups under the influence of the fed factor: the maximum increase over the entire study period was in animals of the 3rd experimental group, which was 2.7% higher compared to analogues from the control group. The results of the T-RFLP analysis showed that an increase in cellulolytic bacteria is already observed in the rumen of 2-month-old calves. In the experimental group, their quantities are almost 2 times greater (P≤0.05) compared to calves in the control group. Based on the data obtained, it can be concluded that the use of feed complexes from essential oils of coriander and fennel usually has a positive effect on the formation of the microbiome of the calf rumen.