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W. Klingmuller Lehrstuhl fur Genetik, Universitat Bayreuth, UniversitatsstraBe 30, 8580 Bayreuth, FRG Growth of higher plants, particularly agricultural crops, de- pends on combined nitrogen. To obtain high yields, combined nitrogen has to be supplied as fertilizer. However, the che- cal production of nitrogen fertilizers is very energy consuming and costly. In underdeveloped countries there is in addition the problem of how to get such fertilizer and how to distribute it. Efforts to replace chemical nitrogen fertilizers by other means of nitrogen fertilization are therefore important. Some bacteria have the capability to use molecular nitrogen from the atmosphere. Such nitrogen is thus transformed into a bound form. Responsible for this capability is a gene group they carry, the nif genes, and an enzyme system, they produce, nitrogenase. An example for such bacteria are rhizobia, which grow in symbioses with leguminous plants, and in their root nodules bind molecular nitrogen. The host plant takes advantage of this. Less well known, but being studied with increasing intensity, are the so called Azospirillum bacteria. They too bind molecular nitrogen. But, in contrast to rhizobia, they do not form nodules on the roots of legumes, but live in loose associations with the roots of other plants, for instance maize, wheat, rice or other grain crops. By exploiting the capabilities of these bacteria it can be hoped to find a way for supplying biologically bound nit- gen to grain crops.