The paddy soil in the Annamalai University Experimental Farm was analysed for the presence ofAzotobacter spp. in the wet and dry seasons.A. chroococcum, A. indicus, A. lacticogenus, A. agilis, A. beijerinckii andA. vinelandii were the species present, in the same order of population strength. Their populations varied greatly with soil moisture content, the maximum number being present in dry soils. A comparison of the nitrogen-fixing capacities of the bacteria indicated that they were all more effective in dry soil than in wet, andA. chroococcum was the most efficient, closely followed byA. indicus andA. lacticogenus. A. chroococcum, A. agilis, A. beijerinckii andA. vinelandii could fix maximum nitrogen in the pH range of 6·0 to 7·0 of the substratum, whereasA. indicus andA. lacticogenus were relatively more active in the pH range of 3·0 to 8·0.