The effect of lanthanum sulphate on five Azotobacter strains (4A7, 10, 5A2, 5A1, and 3A3) isolated from Allahabad soil consists in a considerable increase in the nitrogen fixation, carbon consumption and in the amount of nitrogen fixed in mg per g of carbon consumed. The presence of 200 microM, 300 microM and 400 microM lanthanum sulphate in culture media increases the fixation of nitrogen by all the nitrogen fixers significantly.