Plasmids in Rhizobia: The Role of Nonsymbiotic Plasmids. Jesus Mercado-Blanco. Department of Plant Ecology and Evolutionary Biology, Utrecht University, Sorbonnelaan 16, 3508 TB Utrecht, The Netherlands. Nicolas Toro; Departamento de Microbiologia del Suelo y Sistemas Simbioticos, Estacion Experimental del Zaidin, Consejo Superior de Investigaciones Cientificas, c/ Profesor Albareda 1, 18008, Granada, Spain. MPMI 9:535-545. Accepted 10 May 1996. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1996.