Dozens of legume mutants that form ineffective nodules have been isolated. Some of these mutants are characterized by a defect in the invasion of bacteria into the plant cells or by impaired differentiation of bacteria into bacteroids. However, some form nodules in which the bacteria have differentiated into bacteroids, but which completely lack nitrogenase activity or exhibit a low level of nitrogenase activity. This indicates that nitrogenase activity of symbiotic bacteria in nodules is controlled by host-plant genes. The possible roles of host-plant genes in the regulation of nitrogenase activity of microsymbionts are discussed.