Bacteroidales and Clostridiales are the dominant orders in the rumen community of the Russian Arctic reindeers P­07.2­09 T. Dunyashev I, L. IlinaI, E. YildirimI, D. SobolevI, K. LaishevII IBIOTROF+ Ltd, St. Petersburg, Russia, IINorthwest Center for Interdisciplinary Research of Food Security Problems, St. Peterburg, Russia In the period 2017­2018, a series of expeditions were organized in the Yamalo­Nenets Autonomous District (Kharp, forest­tundra natural and climatic zone) and the Nenets Autonomous District with the aim of colecting rumen content from reindeer of various ages, including calves. A total of 38 samples were taken from reindeer (Rangifer tarandus) of the Nenets breed. The phylogenetic affiliation of the constituents of the microbial community of the reindeer rumen was determined using the NGS 16s rRNA gene. Data analysis was performed in QIIME software. The dominant position was occupied by the phyla Bacteroidetes and Firmicutes. In total, they occupied from 65 to 88% of all bacteria in the reindeer rumen. Representatives of phyla Cyanobacteria took the following position. The proportion of Cyanobacteria increased in the winter. If in the summer period their share was from 0.5 to 3.3%, then in the winter from 1.5 to 12.2%. These differences are significant in the Nenets Autonomous District ­ the proportion of cyanobacteria there was significant in the winter from 6.7 to 12.2% (p = 0.02), while in the Yamalo­Nenets Autonomous District it did not exceed 5 % in the winter. Verrucomicrobia occupied from 0.2 to 4% in the rumen community. Bacteria of other phyla (Tenericutes, SR1, TM7, Spirochaetes, Chloroflexi, Proteobacteria, Elusimicrobia, Actinobacteria, Fibrobacteres, Fusobacteria and others) were less represented in the reindeer rumen community. The core community included representatives of the orders Bacteroidales (from 10 to 40%) and Clostridiales (from 8 to 25%), the proportion of these bacteria was more dependent on the season, so in summer the proportion of Bacteroidales was higher than in winter. This is probably due to a more favorable composition of the summer diet compared to the winter one, in which lichens and shrub branches occupy a large share. This research was supported by a grant from the Russian Science Foundation № 17­76­20026..