Biological nitrogen fixation, the conversion of nitrogen from the atmosphere to ammonia, is the main process of nitrogen input to the earth today. There this nitrogen is indispensable for plant and animal production, and the maintenance of adequate protein standards. This 1976 volume provides information, presented at an international symposium in Edinburgh, on the free-living nitrogen-fixing bacteria and blue-green algae. In addition to information on the distribution of the nitrogenase enzyme within these groups, their role in the soil and in aquatic systems is considered, as are the methods of measuring nitrogen fixation. Particular attention is paid to the biochemistry of the process, much of which has been elucidated using free-living nitrogen-fixing organisms.