Аннотация:In a recent decade a significant reconsideration of the role and scales of microbial nitrogen fixation on Earth has occurred. It has fundamentally changed views at this process. The main concepts of symbiotic and associative nitrogen fixation are commonly linked with plants. The information about the association of animals with nitrogen fixers is still rare. We have studied the representatives of different groups of animals in order to detect diazotrophs and try to estimate their contribution to general nitrogen balance of these animals. The nitrogenase activity was found (by means of measuring N balance, by 15N and C2H2-reduction) in the digestive tract of earthworms, some genus of termites and in insect larvae (click beetles (Elateridae), crane flies (Tipulidae)). In ants’ gut the nitrogenase activity was not detected but it was constantly high in ant-hill material. It was first shown that the interaction between nitrogen-fixing bacteria and arthropods seemed to be an association but not a mutualistic symbiosis.The high bacterial nitrogen fixing activity was revealed in the digestive tract of some vertebrate herbivores (elks (Alces alces), red deer (Cervus elaphus), voles (Microtinae), gerbils (Gerbillinae), hares (Leporidae), pikas (Ochotona), beavers (Castor) and some others). In herbivores’ cecum the quantity of nitrogen fixing bacteria can reach up to 1/3 of total microorganisms number. More than 20 genus of bacteria associated with a digestive tract are active diazotrophs. In order to compensate host's needs in dietary nitrogen diazotrophs could excrete nitrogen-containing compounds in vivo or be a protein feed for the host organism (also by coprophagy). According to our estimates the contribution of microbes to the nitrogen balance of animals can be physiologically significant.