Research in the field of Rhizobium-legume symbiosis faces a new challenge: integrate the wealth of information generated by genomic projects. The goal: apprehend the complexity of the molecular mechanisms involved in symbiotic associations. At the time of writing, the genomes of three micro-symbionts (Bradyrhizobium japonicum, Mesorhizobium loti and Sinorhizobium meliloti) have been sequenced, and two more (those of Rhizobium leguminosarum and Rhizobium etli) will be completed in the near future. Together, completed rhizobial genomes represent already 23,393,822 bp of DNA sequence and 21,797 predicted open reading frames (ORFs). To identify candidate-symbiotic genes in such a broad database, predict their function and dissect the regulatory networks that govern their expression are no simple tasks. One way to confront this problem is to combine different datasets, in particular genetic and transcriptional maps as well as predicted promoters from bioinformatics analyses. Here, we would like to illustrate this type of approach with the analysis of the symbiotic plasmid (pNGR234a) of the broad host-range Rhizobium sp. NGR234.