A dinitrogen fixation by bacteria living in symbiosis with leguminous plants is a widespread and well-known phenomenon. However, also some free-living bacteria, frequently entering into associations with various plants, reveal capabilities of fixing molecular nitrogen. Such endophytes include bacteria like Acetobacter diazotrophicus and Herbaspirillum seropediceae, discovered in countries of the tropical climate, where they lived in associatins with plants characteristic for this climate as: sugar cane, sorgo and grasses. Until now, however, very little is known about the occurrence of these bacteria in the climate of the temperate zone. Therefore, the objective of this experiment was to find out the influence of the inoculation of Italian ryegrass, Bromus willdenowii and Festulolium braunii with Acetobacter diazotrophicus and Herbaspillirum seropedieae on N2 fixation.