Reindeer are a unique animal species that have adapted to the harsh Arctic conditions. Adaptive features of the structure of the gastrointestinal tract of reindeer allows you to eat actively digest and assimilate lichens, foliage, shrubs, snowy greens. Along with beneficial microflora, pathogenic and conditionally pathogenic microorganisms live in the reindeer rumen. The purpose of our research is to study and compare the features of the rumen microbiome in healthy and sick reindeer with necrobacteriosis. The subject of the study was samples of cicatricial contents taken from 38 reindeer (Rangifer tarandus) of the Nenets breed of different age groups. Microbiome studies were performed by NGS sequencing on the MiSeq next generation sequencing platform (Illumina, USA) using primers for the V3-V4 region of 16S rRNA.Based on the studies, it was possible to establish that the microbiome of the reindeer rumen in summer and winter is characterized by certain differences. During the summer grazing period, the presence of a greater number of pathogenic bacteria such as Fusobacterium sp., Parvimonas sp. and Porphyromonas endodontalis, while in the spectrum of pathogenic microorganisms, bacteria of the genus Fusobacterium, which are considered the main infectious agent in necrobacteriosis, were most often isolated. In the same period, the presence of bacteria of the genus Sharpea, belonging to the phylum Erysipelotrichaceae, which ferment sugars with the formation of lactic acid and can cause the development of lactic acidosis, was established in deer rumen samples. In calves, during the disease, the number of chytridiomycete fungi, which are involved in the fermentation of polysaccharides, decreases.