An economically important problem in microbial ecology concerns the efficacy of rhizobial inoculants for the formation of nitrogen-fixing root nodules on legume crop plants such as soybean, alfalfa, and clover. Some strains of rhizobia can increase symbiotic nitrogen fixation under controlled conditions. However, attempts to improve nitrogen fixation under agricultural conditions with such strains often fail, usually as a result of the presence of indigenous rhizobia limiting nodulation by the inoculum strains. This problem is referred to as the Rhizobium competition problem, and molecular genetics is being used to address the problem from two perspectives. First, the host specificity of rhizobia is being characterized with the long term goal of developing strains that can nodulate a very strain-specific host-legume genotype. Second, the genetic basis of competitiveness in several strains is being examined. Genetic determinants of nodulation competitiveness have been isolated and mechanisms for their stable integration into the genome of superior nitrogen-fixing strains have been developed. Several phenotypes have been identified as playing an important role in nodulation competitiveness including antibiosis, motility, speed of nodulation, cell-surface characteristics, and nodulation efficiency. Several solutions to this problem are likely to result from these strategies and will be useful for certain legumes in specific locations.