The effects of antibiotic production on rhizosphere microbial communities of field-grown Phaseolus vulgaris were assessed by using ribosomal intergenic spacer analysis. Inoculum strains of Rhizobium etli CE3 differing only in trifolitoxin production were used. Trifolitoxin production dramatically reduced the diversity of trifolitoxin-sensitive members of the alpha subdivision of the class Proteobacteria with little apparent effect on most microbes.