The 12th International Congress on Nitrogen Fixation was held in Foz do Iguacu, Parana State, Brazil, from the 12th to 17th of September, 1999. This volume constitutes the proceedings of this Congress and represents a compilation of the presentations by scientists from 38 countries who came to discuss the progress made, to exchange views and to collaborate. Since the first meeting in Pullman, Washington, USA, in 1974, this series of Congresses has been held three times in the United States, four times in Western Europe, in Australia, Mexico and Russia, and now for the first time in South...