Nodulation-defective rhizobia and their nodule-forming derivatives containing cloned DNA from the wild type were used to study nodulation suppression in Phaseolus vulgaris L. Non-nitrogen-fixing derivatives which formed rhizobia-containing white nodules induced partial suppression. Comparison of this with the complete suppression by Fix derivatives and a Fix mutant which formed rhizobia-containing pink nodules suggests that the extent of suppression may be related to successive stages of nodule development.