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Introduction Quantification of biological nitrogen fixation Sugar cane Wetland rice Plant-associated N2-fixing bacteria Sugar cane Wetland riceProspects for the future Acknowledgements References Abstract 15N isotope and N balance studies performed over the last few years have shown that several Brazilian varieties of sugarcane are capable of obtaining over 60% of their nitrogen (>150 kg N ha-1 year-l) from biological nitrogen fixation (BNF). This may be due to the fact that this crop in Brazil has been systematically bred for high yields with low fertilizer N inputs. In the case of wetland rice, N balance experiments performed both in the field and in pots suggest that 30 to 60 N ha-l crop-1 may be obtained from plant-associated BNF and that different varieties have different capacities to obtain N from this source. 15N2 incorporation studies have proved that wetland rice can obtain at least some N from BNF and acetylene reduction (AR) assays also indicate differences in N2-fixing ability between different rice varieties. However in situ AR field estimates suggest plant-associated BNF inputs to be less than 8 kg N ha -1 crop -l. The problems associated with the use of the 15N dilution technique for BNF quantification are discussed and illustrated with data from a recent study performed at EMBRAPA-CNPAB. Although many species of diazotrophs have been isolated from the rhizosphere of both sugarcane and wetland rice, the recent discovery of endophytic N2-fixing bacteria within roots, shoots and leaves of both crops suggests, at least in the case of sugarcane, that these bacteria may be the most important contributors to the observed BNF contributions. In sugarcane both Acetobacter diazotrophicus and Herbaspirillum spp. have been found within roots and aerial tissues and these microorganisms, unlike AzospiriUum spp. and other rhizospheric diazotrophs, have been shown to survive poorly in soil. Herbaspirillum spp. are found in many graminaceous crops, including rice (in roots and aerial tissue), and are able to survive and pass from crop to crop in the seeds. The physiology, ecology and infection of plants by these endophytes are fully discussed in this paper. The sugarcane/endophytic diazotroph association is the first efficient N2-fixing system to be discovered associated with any member of the gramineae. As yet the individual roles of the different diazotrophs in this system have not been elucidated and far more work on the physiology and anatomy of this system is required. However, the understanding gained in these studies should serve as a foundation for the improvement/development of similar N2-fixing systems in wetland rice and other cereal crops.