Diazotrophic cyanobacteria are photoautotrophic organisms that require sunlight as a sole energy source for the fixation of carbon and nitrogen. Therefore, they have great potential as biofertilizers, and their use will decrease fuel demand for fertilizer production. The agronomic potential of heterocystous cyanobacteria, either free-living or in symbiotic association with water fern Azolla, has long been recognized. This has led to the development of small scale biotechnology involving the use of paddy soils with appropriate cyanobacterial strains as biofertilizers in rice culture, as has been reported from China, Egypt, Philippines, and India. Besides increasing soil fertility and sustaining rice yield, these forms are also reported to benefit rice seedlings by producing growth-promoting substances, the nature of which is said to resemble gibberellins. Whereas the incorporation of nif genes into the rice plants by using tissue culture and modern genetic tools remain one of the ambitious research goals, the use of cyanobacterial diazotrophic technology in rice agriculture offers an immediate or even long-term alternative to synthetic nitrogen fertilizers, particularly in developing countries and the world as a whole. However, one of the weaknesses in this technology is the heavy application of several toxic agrochemicals, especially herbicides, which are reported in mostmore » cases as inhibitors of cyanobacterial diazotrophic growth, and in some cases as mutagenic. Naturally, a successful biotechnology requires the selection of suitable diazotrophic strains, as biofertilizers, that could tolerate the field-dose concentrations of herbicides and secrete ammonia.« less