Technical Abstract: Leguminous crops are the best known dinitrogen (N2) fixing systems and are becoming increasingly important in attempts to further develop sustainable agriculture systems. Soybean plays a major role in N2 fixation since it is the most widely grown legume crop with 69.4 million ha grown world-wide in 1997-98. Methods of enhancing N2 fixation continue to be of major importance in efforts to decrease N fertilizer use and minimize ground water pollution. Dinitrogen fixation is also important in enhancing seed protein production, and in contributing N for succeeding crops under some conditions. For N2 fixation to contribute N to succeeding crops, the Pfix (percentage of total plant nitrogen derived from N2 fixation) must exceed the N-harvest index of the crop. To meet the goal of making agriculture systems more sustainable, it is of importance to maximize use of N2 fixing systems to maintain crop yields while minimizing impact on the environment from loss of N from the soil system. In spite of major effort to enhance symbiotic N2 fixation, it is not desirable to develop legume systems which are so dependant on symbiotically fixed nitrogen that they do not utilize available soil nitrogen. This scenario would be detrimental to agriculture because available soil N not used by the crop will be subject to loss from the environment. Effort must be expended on the roles of both the micro- and macro-symbiont of a given symbiotic relationship, and to define the niche that these symbiotic systems occupy in agricultural productivity and sustainability. It will, therefore, require effort by researchers from several disciplines to enhance the symbiotic partnership and at the same time minimize N losses from the environment.