The symbiosis between soil bacteria (rhizobia) and legumes is an integral component of sustainable agriculture, particularly in Australia where our generally nutrient-poor soils are a challenge to maintaining productive farming systems. All legumes currently in use in Australian agriculture are exotic, and their rhizobia have similarly been introduced, either deliberately or inadvertently, from other parts of the world. Rhizobia of the genus Mesorhizobium are of particular interest, as they form symbiotic relationships with important crops such as chickpea (Cicer arietinum) and the pastures Biserrula pelecinus and Lotus. Mesorhizobium spp. are known to harbour symbiotic genes on large chromosomally-encoded mobile genetic elements or “symbiosis islands”. Mesorhizobial symbiosis islands were first discovered by the Ronson laboratory in New Zealand who demonstrated in situ transfer of an approximately 500 kbp chromosomal region encoding nodulation and nitrogen fixation genes from the inoculant M. loti strain to non-symbiotic soil mesorhizobial species. An almost identical phenomenon was subsequently discovered in Australian soils by the Howieson and O’Hara laboratories where the newly introduced pasture legume, B. pelecinus, which lacks native symbionts in Australian soils, was inoculated with the efficient nitrogen-fixing strain M. ciceri bv. biserrulae WSM1271. Analysis of B. pelecinus nodule occupants 6 years after inoculation revealed that in situ transfer of a symbiotic island from WSM1271 to previously non-symbiotic indigenous mesorhizobia had occurred, with these new nodule bacteria accounting for 5% of the nodules surveyed. Importantly, symbiotic testing showed that the novel isolates are less effective nitrogen-fixing microsymbionts than the original inoculant. Phylogenetic analysis and full genome sequencing of the inoculant strain WSM1271 and two novel isolates WSM2073 and WSM2075 revealed that all three strains are different Mesorhizobium spp. In contrast to M. loti R7A, WSM1271 appears to harbour a novel tripartite symbiosis island, which is capable of transfer and which has been acquired by the recipient strains WSM2073 and WSM2075 (See Haskett et al., these proceedings). WSM1271 also carries a small accessory plasmid which sequence analysis suggests may encode symbiotic functions (see Brewer et al., these proceedings). The symbiotic data, structural analysis of the symbiosis islands and the relevance of this work to the development of stable inoculants for agriculture will be discussed.