Rangifer tarandus (reindeer) – is actively bred in the northern regions of different countries. Therefore, an urgent task is to deepen information about the features of reindeer adaptations. Rumen symbiotic microorganisms play an important role in the life of Rangifer tarandus, allowing animals to efficiently use scarce nutrient resources of the tundra and forest-tundra. The microbial community of the reindeer rumen, as well as its age-related changes, are the least studied compared to other ruminants. The comparative analysis results of rumen bacterial community composition of calf (4 months), young animals (1–2 years) and adults (3–6 years) Rangifer tarandus of the Russian Arctic are presented for the first time. The reindeer ruminal bacterial community composition was analyzed in the laboratory of the ‘BIOTROF+’ Ltd by T-RFLP method. In the ontogenesis, significant changes in the microorganism representation were noticed, the greatest of which was noted in microorganism involved in carbohydrate fermentation. The content of cellulolytic Clostridia and the acid-utilizing species of the Negativicutes (P is less than 0.05) decreased with age, but bacteria with the amylo- and cellulosolytic properties of the phylum Bacteroidetes increased (P is less than 0.05). A wide range of microorganisms which traditionally belong to the pathogens of various animals and humans diseases was revealed. With age, a tendency to increase the number of pathogens, including the bacteria of the families Campylobacteraceae, Burkholderiaceae, phylum Fusobacteria, and the genus Staphylococcus was noticed. The greatest percent of opportunistic microorganisms, including phylum Actinobacteria and the family Enterobacteriaceae, were detected in young animals.