Azotobacter chroococcum was grown on cultures containing five carbon sources alone and also in co-cultures with three cellulolytic fungi (Aspergillus niger, Penicillium funiculosum and Trichoderma harzianum). In the absence of fungal species, nitrogenase activity was relatively low. The best nitrogenase activity was recorded in cultures containing faba bean straw followed by that in cultures havingwheat straw, sugar cane leaves, carboxymethyl cellulose (CMC) or cellulose. In co-cultures with fungi, Azotobacter showed substantial nitrogenase activity on all tested substrates. Azotobacter-Trichoderma association showed thg highest nitrogenase activity.