Nine flavonoid aglycones released from black bean (Phaseolus vulgaris 'PI165426CS') seeds and roots induced nodC::lacZ transcription in Rhizobium leguminosarum bv. phaseoli strains containing extra cloned copies of the regulatory genes nodD1, nodD2, or nodD3 from that biovar. Individual flavonoids generally induced highest levels of nodC::lacZ transcription (Imax) with extra copies of nodD2, and the concentration required for half-maximum induction (I50) was lowest with extra copies of nodD1 genes. One apparently unique feature of R. l. bv. phaseoli is that naturally released flavonoids with very diverse structures induce nod genes. For all three nodD genes, two compounds exuded from roots, genistein and naringenin, produced much higher levels of nodC::lacZ transcription than other flavonoids, but this fact was not explained by increased transcription of the nodD genes themselves. The remaining seven flavonoid aglycones showed reproducibly different capacities to induce nodC::lacZ transcription, but all were considerably less powerful inducers than genistein and naringenin in strains with extra copies of each of the nodD genes. Tests with glycosides of the nod-gene inducers showed that glycosides, which are normally released by bean, had lower I50 values than the corresponding aglycones with all nodD genes. Additive interactions observed between the strong nod-gene inducer genistein and the weak inducer eriodictyol remain to be explained at the molecular level.