Infusoria from the digestive tract of herbivorous mammals have been known to science for almost a century and a half. However, protozoans remain an understudied group of rumen dwellers in ruminants. In cattle, in a multi-chambered stomach, in addition to microorganisms (fungi and bacteria), there are a large number of protozoa (infusoria), which also participate in the digestive processes that support the vital activity of the body. Endobiontic infusoria take an active part in the digestion of complex carbohydrates, fiber, in the breakdown of indigestible polysaccharides, which are essential sources of energy for animals. In this regard, the study of the species and quantitative composition of the infusoria fauna of cattle bred in the Far North, as one of the factors of sustainable adaptation to extreme conditions, is of great scientific and practical interest.This article presents the results of a study of endobiontic infusoria of cattle breeds bred in Yakutia. A total of 36 species have been identified and described in 12 genera and 3 families. It was found that the genus Entodinium and the genus Isotricha were the most represented in the Yakut cattle, while the genus Entodinium and Euplodiniumin are in the crossbred and cultivated cattle. The smallest genus in all groups are Polyplastron and Metadinium.