Many, though not all, non-heterocystous cyanobacteria can fix N-2. However, very few strains can fix N-2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N-2 fixation by laboratory cultures and natural populations of non-heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N-2 fixation to environmental factors such as fixed nitrogen, O-2 and the pattern of illumination. The integration of N-2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of nonheterocystous cyanobacteria are highlighted. [KEYWORDS: non-heterocystous cyanobacteria; N-2 fixation; nitrogenase; immunolocalization; ATP and reductant; diurnal rhythm; natural environment Aerobic nitrogen-fixation; filamentous nonheterocystous cyanobacterium; synechococcus sp rf-1; marine microbial mat; anabaena-variabilis atcc-29413; plectonema-boryanum pcc-73110; circadian gene-expression; sp strain pcc-6803; sp pcc 6909; unicellular cyanobacterium]