Several gram-negative plant and animal pathogenic bacteria have evolved a type III secretion system (TTSS) to deliver effector proteins directly into the host cell cytosol. Sinorhizobium fredii USDA257, a symbiont of soybean and many other legumes, secretes proteins called Nops (nodulation outer proteins) into the extracellular environment upon flavonoid induction. Mutation analysis and the nucleotide sequence of a 31.2-kb symbiosis (sym) plasmid DNA region of USDA257 revealed the existence of a TTSS locus in this symbiotic bacterium. This locus includes rhc (rhizobia conserved) genes that encode components of a TTSS and proteins that are secreted into the environment (Nops). The genomic organization of the TTSS locus of USDA257 is remarkably similar to that of another broad-host range symbiont, Rhizobium sp. strain NGR234. Flavonoids that activate the transcription of the nod genes of USDA257 also stimulate the production of novel filamentous appendages known as pili. Electron microscope examination of isolated pili reveals needle-like filaments of 6 to 8 nm in diameter. The production of the pili is dependent on a functional nodD1 and the presence of a nod gene-inducing compound. Mutations in several of the TTSS genes negate the ability of USDA257 to elaborate pili. Western blot analysis using antibodies raised against purified NopX, Nop38, and Nop7 reveals that these proteins were associated with the pili. Mutations in rhcN, rhcJ, rhcC, and ttsI alter the ability of USDA257 to form nodules on Glycine max and Macroptilium atropurpureum.