In association with legumes, symbiotic nitrogen fixation by rhizobia is a major contributor to the combined pool of assimilable nitrogen in the biosphere. It takes place in specialized root- or stem-organs called nodules, in which large polyploid nodule cells house thousands of endosymbiotic nitrogen-fixing bacteroids. Our model system, Sinorhizobium sp. strain NGR234, is a promiscuous bacterium capable of forming nodules on more than 150 genera of legumes and fixing nitrogen in many nodules of determinate type (DNs) as well as indeterminate type (IDNs). By combining microscopy and cytology we have examined the fate of bacteroids of NGR234 in distinct host backgrounds. The regulatory networks that govern gene expression in bacteroids were also explored, particularly the BxqH-regulon which is under the control of a LuxR-like regulator. Finally we also examined the role in symbiosis of a second type-three secretion system (T3SS-II) that was identified during the analysis of the NGR234 genome.