Lotus purshianus (Benth.) Clem, and Clem., growing on a copper mine waste in Northern California, exhibits copper tolerance. Effective nitrogen fixation (acetylene reduction) is seen in copper-enriched soils. When subcultured in liquid culture, Rhizobium loti isolated from root nodules of L. purshianus growing on the copper mine, showed considerably greater copper tolerance than did rhizobium isolated from plants growing in a nearby field. No difference was detected in either the pattern of copper uptake or concentration in plant tissue between tolerant and non-tolerant L. purshianus. However, a copper accumulation mechanism associated with copper tolerance was found in the symbiotic rhizobium. The successful colonization of copper enriched soil by this legume species is accomplished by the evolution of copper tolerance in both the legume plant and its symbiont. The copper tolerance of the plant and rhizobium may have evolved independently.