Rhizobium meliloti proved to be sensitive to low concentrations of the herbicide phosphinothricintripeptide (PTT) and its active ingredient phosphinothricin (PT), which was formerly assumed to be nontoxic for most of the bacteria analysed. Growth was more strongly reduced in sterile synthetic media and less reduced in sterile soil; in unsterile soil only a transient growth reduction was detectable. Sensitivity was also observed in five out of eight other species analysed. In all sensitive species tested, spontaneous resistances to PT occurred. Under sterile conditions, PTT and PT reduced rhizobial nodulation rates of PT-resistant alfalfa plants drastically; however, nitrogen fixation in the few nodules that arose was unaffected. Because of the small number of nodules, the overall fixation rate was strongly diminished. In unsterile soil, nodulation and nitrogen fixation rates were not changed, possibly because of the rapid degradation of PTT and PT in the soil. Using a herbicide as model substance it could be demonstrated that the sensitivity of R. meliloti to chemical additives in the soil can be detected by analysing its growth rate, and that even a weak impact carl influence its nodulation capacity. R. meliloti has proven to be a fast, easy and sensitive detection system for bacteriostatic components present in the soil.