The soil bacterium Rhizobium meliloti invades and establishes a symbiosis with host plants such as alfalfa. Bacterial nodulation (nod) genes are required for this invasion, but their mechanism of action and the timing of their expression are not known. We have used translational lacZ fusions to monitor expression of nodD and nodC, which are located in the cluster of four nod genes on the R. meliloti megaplasmid (pSym). nodD is expressed at comparable levels by broth-grown bacterial cells and by cells exposed to exudates from aseptically grown plants. Activity of the nodC-lacZ protein fusion in broth-grown bacterial cells is very low. nodC-lacZ activity is increased approximately equal to 30-fold by plant exudate when nodD is expressed at a high level but not when nodD expression is low. Both fusions show differences in expression when borne on inc-P vectors as compared to when located on the pSym megaplasmid. nodD expression from vector-borne copies of the nod segment and response of nodC to plant exudate appear to require additional loci on the megaplasmid. Our results suggest that regulation of bacterial nod gene expression is an important control mechanism early in the symbiosis, and that the biochemical nature of some nod gene products may be cryptic except in cells grown in the presence of plant exudate.