The best inducers of nod::lacZ translational fusions in Bradyrhizobium japonicum are isoflavones, primarily genistein and daidzein. Upstream of the nodABC genes in B. japonicum is a novel gene, nodY, which is coregulated with nodABC. Measurements of the activity of lacZ fusions to the nodD gene of B. japonicum show that this gene is inducible by soybean seed extract and selected flavonoid chemicals. The induction of the nodY ABC and nodD operons appears to require a functional nodD gene, indicating that the nodD gene product controls its own synthesis as well as other nod genes.