The Instituto Internacional de Estudios Avanzados, founded by decree of the President of the Republic of Venezuela, Dr. Luis Herrera Campins, on November 15, 1979 and constituted on July 8, 1980, sponsors, within the Educational Program for the Third World Coun- tries, a series of courses on Scientific Basis for Agricultural Pro- ductivity, the first of which BIOLOGICAL NITROGEN FIXATION AND ITS ECOLOGICAL BASIS, was held in Caracas on January 18-29, 1982. We are pleased to present, in this volume, the proceedings of this course. It is a pleasure to thank each and everyone of the persons who collaborated in the realization of this event, espe- cially: Dr. Martin Alexander (Cornell University, U.S.A.); Dr. Luis Bascones (FUSAGRI, Venzuela) and Dr. Ivan Casas (Universirlad del Zulia, Venezuela), who assisted me during the preparation and exe- cution of this activity; the teaching staff, whose presentations are the chapters of this volume; and the participants from Argentina, Brazil, Colombia, Costa Rica, Egypt, Ghans, Guatemala, India, In- donesia, Mexico, Nicaragua, Nigeria, Pakistan, Peru, Syria, and Venezuela who shared this course with us. Special recognition is given to UNESCO and FUSAGRI for their valuable contribution. I sincerely hope this publication will be the first of a series on the same level that will originate from the courses in this same program.