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Dryland salinity is an increasing problem in Australia, due to the rising levels of salt-laden water tables. The primary cause for the increase in salinity is thought to be the replacement of native perennial vegetation in agricultural areas with annual crops and pastures that transpire less water. Greater use in agricultural systems of deep-rooted perennials, which have greater water requirements than annuals, will help to lower the water table and prevent further salinisation of soils. Lucerne (Medicago sativa ) is the principal perennial pasture legume targetted to overcome dryland salinity in southern Australia. However, the acid sensitivity of Sinorhizobium , the microsymbiont of lucerne, limits M. sativa nodulation and establishment in many saline regions. Rhizobium tropici is a microsymbiont which is low pH tolerant; however it does not form an effective symbiosis with lucerne. This work aims to develop a strain of R. tropici which is both acid tolerant and able to effectively nodulate M. sativa , thus extending the range of soils in which lucerne can be grown. The megaplasmids of S. medicae WSM419 required for an effective symbiosis with lucerne were marked and transferred into R. tropici via Agrobacterium tumefaciens . R. tropici transconjugants containing either pSymA or pSymB from WSM419 were inoculated onto M. sativa seedlings. One of the R. tropici strains containing pSymA showed an enhanced ability to nodulate M. sativa , but the nodules were ineffective at fixing nitrogen. Current work is focussed on transferring both WSM419 megaplasmids into R. tropici , in an effort to obtain an acid-tolerant microsymbiont that can fix nitrogen with M. sativai.