The dinitrogen fixation activity ofAzospirillum sp., andPantoea agglomeransstrains was determined by15N2incorporation after incubation with15N2labeled air or/and by acetylene reduction. These bacterial strains were able to fix N2both in pure culture and in association with wheat plants in hydroponics. Nitrogenase activity ofAzospirillum sp., in pure culture was more rapidly inhibited by the addition of NH4+than NO3−. The N2fixation ofP. agglomeransdecreased only by NH4+-addition, but was stimulated by NO3−. Nitrogen fixation in association with wheat plants remained unaffected by both N compounds. However, nitrogen derived from the atmosphere (Ndfa) contributed only very little to the overall nitrogen nutrition of the plants.