This chapter focuses on genetic techniques in Rhizobium meliloti. Rhizobia have been studied extensively because of their ability to form nodules on the roots of leguminous plants and fix atmospheric nitrogen. Many genetic techniques have been developed to facilitate analysis of several species and strains of Rhizobium. Rhizobium meliloti strain SU47, an alfalfa symbiont, has one of the most advanced genetic systems. Two basic methods are used to transfer DNA into and out of R. meliloti in the course of genetic manipulations—transduction and conjugation. Although it is possible to mutagenize R meliloti with chemical mutagens, such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), transposon mutagenesis has proved to be a very useful tool because mutations induced by transposons are genetically marked by the antibiotic resistance of the transposon and physically marked by the presence of the transposon.