Root exudates of different pearl millet varieties showed quantitative differences in organic carbon, reducing sugars, total and amino nitrogen. The growth and nitrogenase activity ofAzospirillum were stimulated by the addition of root exudates in the culture medium. Root exudates were also found to support the growth ofAzospirillum in the rhizosphere. Inoculation withAzospirillum significantly enhanced the root exudation in axenically grown pearl millet plants accompanied by an increase in the permeability of roots. N2-ase activity of the inoculated plants differed among the varieties and was related to the amount of organic carbon released in the exudates. Addition of extraneous carbon source significantly increased the nitrogenase activity of the roots as the carbon compounds provided by the root exudates appear to be inadequate for the optimum expression of nitrogenase activity. The implications of these results in the pearl millet-Azospirillum association were discussed.