Nodulation and nitrogen fixation genes of Mesorhizobium loti are encoded on the chromosome of the bacterium. Nevertheless, there is strong evidence that these genes can be transferred from an inoculant strain to nonsymbiotic mesorhizobia in the field environment. Here we report that the chromosomal symbiotic element of M. loti strain ICMP3153 is transmissible in laboratory matings to at least three genomic species of nonsymbiotic mesorhizobia. The element is 500 kb in size, integrates into a phe-tRNA gene, and encodes an integrase of the phage P4 family just within its left end. The entire phe-tRNA gene is reconstructed at the left end of the element upon integration, whereas the 3' 17 nucleotides of the tRNA gene are present as a direct repeat at the right end. We termed the element a symbiosis island on the basis of its many similarities to pathogenicity islands. It may represent a class of genetic element that contributes to microbial evolution by acquisition.