Some plant species establish mutualistic cooperation with nitrogen-fixing bacteria to overcome nitrogen shortage. The interaction between legume plants and rhizobial bacteria is the canonical example of such mutualism. It leads to the formation of root nodules, which provide the environment for the bacteria to convert atmospheric dinitrogen into ammonia. Nodule formation is initiated by rhizobium-secreted compounds called nodulation (Nod) factors. Perception of Nod factors by receptors in the plant root cells initiates nodule organogenesis and is essential for bacterial infection (1). Several legumes evolved a narrow host range such that only one or a few rhizobium species can initiate nodulation (2). This specificity is largely determined by the Nod factor receptors. On page 663 of this issue, Bozsoki et al. (3) provide structural characterization of the binding site of a Nod factor receptor in legumes. The structural basis of rhizobial Nod factor recognition is a key to understanding the evolution of specificity in symbioses.