Nodulins are organ-specific plant proteins induced during symbiotic nitrogen fixation. Nodulins play both metabolic and structural roles within infected and uninfected nodule cells. In soybean, several nodulin genes, coding for abundant nodulins, have been identified and isolated. Structural analysis of some of these genes has revealed their possible mode of regulation and the subcellar location of the protein product. Studies of ineffective symbiosis based on cultivar-strain genotype differences suggested that both partners influence the expression of nodulin genes. Concomitant with nodule organogenesis, the Rhizobium undergoes substantial differentiation leading to the accumulation of nodule-specific bacterial proteins, bacteroidins. The major structural alteration occuring in the infected cell is the formation of a membrane enclosing the bacteroid (peribacteroid membrane). A number of nodulins are specifically targetted to this membrane during endosymbiosis. The induction of nodulins and bacteroidins leads to the formation of an effective nodule. Nodulin genes can be induced in vitro by factors derived from nodules suggesting that trans-activators may be involved in derepression of the host genes necessary for Rhizobium-legume symbiosis.