[en] Alternatives to the use of commercial N fertilizers must therefore be sought if we are to sustain crop production. Plant species such as Azolla which in symbiosis with the blue-green alga Anabena azollae are capable of converting atmospheric N₂ into usable forms through the microbial process of biological N₂ fixation. Such nitrogen fixing systems offer an attractive and ecologically sound means of reducing external inputs of chemical N fertilizers in cropping systems. Azolla is a free-floating water fern widely distributed in aquatic habitats of the tropics and sub-tropics. Because of its aquatic nature, Azolla is of particular value to flooded rice. Several methods have been used to estimate biological N₂ fixation of Azolla. In a Co-ordinated Research Programme on Azolla executed by the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, quantitative integrated values for biological N₂ fixation by Azolla have been obtained through use of the ¹⁵N isotope. Azolla can derive as much as 70% or more of its N from N₂ fixation, yielding 22-24 kg N/ha in about one month. Differences however in N₂ fixation are common depending on the Azolla species or strain and the environmental conditions. In this research programme, we also used ¹⁵N as a tracer to assess the N recovery by rice from Azolla and N balance in the rice cropping system. Incorporation of ¹⁵N labelled Azolla into soil gave an ¹⁵N recovery by rice of 40-50%. On an overall basis under a wide range of environmental conditions in six countries, the percentage ¹⁵N recovery by rice as well as the rice yield increases from Azolla application were similar to those obtained from urea. The general conclusion from this programme is that Azolla is as good as urea as a source of N for rice. There is no more important crop in the world today than rice. It occupies 90% of the area under cereals in the Asia and Pacific region. Research aimed at developing improved integrated fertilizer management practices involving the use of green manures such as Azolla with reduced inputs of chemical fertilizers, is very much needed. 27 refs, 4 figs, 1 tab