Abstract: A simple, generalized method is disclosed for screening a collection of Rhizobium or Bradyrhizobium strains for variants which, in nodulation, have competative abilities above or below a particular experimentally-set criterion. This method involves mixing a Fix.sup.+ test strain with a Fix.sup.- reference strain, followed by inoculation of a legume plant with the mixture and observation of the inoculated plant's foliage for symptoms of nitrogen sufficiency (green foliage) or nitrogen deficiency (yellow foliage). The method is exemplified by the identification of mutants of both R. fredii and B. japonicum which are phenotypically Comp.sup.- on soybeans (Glycine max).