On account of its energetic fixation of nitrogen in culture media, _Azotobacter_ has attracted the greatest attention of workers. The evidence seems to be consistent with the view that _Azotobacter_ is of importance in the soil. Thus the distribution of _Azotobacter_ would appear to be world-wide. It is found all over Western Europe and the United States. Lipman and Burgess[45] found it in soils collected from Italy and Spain, Smyrna, Cairo, the Fayum, the Deccan in India, Tahiti, Hawaii, Mexico, Guatemala, and Canada. C. M. Hutchinson[29] found it to be distributed throughout India. It was found by Omelianski[55] to be widely distributed in European and Asiatic Russia, and by Groenewege[28] in Java. Ashby[1] at Rothamsted, isolated it from soils from the Transvaal, East Africa, and Egypt. Also, an association has sometimes been found between the ability of a soil to fix nitrogen and the occurrence and vigour of its _Azotobacter_ flora. Thus Lipman and Waynick[46] found that if soil from Kansas were removed to California, its power to produce a growth of _Azotobacter_, when inoculated into a suitable medium, was lost, and, at the same time, its nitrogen-fixing power was greatly reduced. Moreover, it is known that conditions favourable to the fixation of nitrogen by _Azotobacter_ in cultures on the whole favour nitrogen fixation in soils. The conditions that favour other aerobic nitrogen-fixing bacteria are, however, not sufficiently distinct to make such evidence of great value. It is usually found that nitrogen fixation is most active in well-aerated soil. Thus Ashby,[1] at Rothamsted, found the nitrogen-fixing power of a soil to decrease rapidly with depth. Similar results were obtained in Utah by Greaves. This suggests, at first sight, that anaerobic nitrogen fixers are unimportant under normal soil conditions. It is, however, quite possible that they may assume an importance when acting in conjunction with aerobic organisms. Thus Omelianski and Salunskov[55] found that beneficial association, or symbiosis, could occur between _Azotobacter_ and _Clostridium pasteurianum_, the former absorbing oxygen from the surroundings, and thus creating a suitable anaerobic environment for the _Clostridium_.