AbstractThe Rhizobium leguminosarum bv. trifolii Sym plasmid pRtr5a was transferred by conjugation to R. meliloti strains deleted or mutated in the nod and fix genes, and also to a plasmid‐less strain of Agrobacterium tumefaciens. The plasmid transfer led to the elimination of the resident cryptic plasmid of the R. meliloti recipients. The Sym plasmid of the recipients was only eliminated in the case of a deletion. In some cases the transferred plasmid was instable. The nod and nif/fix genes of R. leguminosarum bv. trifolii were expressed in R. meliloti and somewhat delayed in A. tumefaciens. A high level of nitrogen fixation on clover was only found in R. meliloti containing pRtr5a. However, this occurred only if the nif/fix genes of the recipient had been deleted before. Inoculation of clover with this R. meliloti strain resulted in a higher dry matter production in comparison to clover inoculated with the donor strain.