Aims: To study the genetic basis of C-4- dicarboxylate transport ( Dct) in relation to symbiotic nitrogen fixation in Mesorhizobium ciceri. Methods and Results: A Tn5- induced mutant strain ( TL16) of M. ciceri, unable to grow on C-4- dicarboxylates, was isolated from the wild- type strain TAL 620. The mutant lacked activities of the enzymes, which use C4- dicarboxylates as substrate. The sequencing of the 3.2kb EcoRI fragment, which was the site of Tn5 insertion, revealed three complete and two partial open reading frames. In the mutant, Tn5 interrupted the rpoN gene, of which only one copy was there. Complementation and biochemical studies suggest that the M. ciceri rpoN activity is required for C4- Dct, maturation of bacteroids and symbiotic nitrogen fixation. The fine structure of the ineffective nodules produced by TL16 on Cicer arietinum L changed in comparison with those produced by the wild type. Conclusions: The mutant strain TL16 suffered a disruption in the rpoN gene. Only one copy of rpoN gene is present in M. ciceri. The mutation abolishes Dct activity. It additionally abolishes the symbiotic nitrogen fixation activity of the bacteroids in the nodules. Significance and Impact of the Study: This first document in M. ciceri shows that a functional rpoN gene is essential for the transport of dicarboxylic acids and symbiotic nitrogen fixation. To study the genetic basis of C-4-dicarboxylate transport (Dct) in relation to symbiotic nitrogen fixation in Mesorhizobium ciceri. A Tn5-induced mutant strain (TL16) of M. ciceri, unable to grow on C-4-dicarboxylates, was isolated from the wild-type strain TAL 620. The mutant lacked activities of the enzymes, which use C-4-dicarboxylates as substrate. The sequencing of the 3.2kb EcoRI fragment, which was the site of Tn5 insertion, revealed three complete and two partial open reading frames. In the mutant, Tn5 interrupted the rpoN gene, of which only one copy was there. Complementation and biochemical studies suggest that the M. ciceri rpoN activity is required for C-4-Dct, maturation of bacteroids and symbiotic nitrogen fixation. The fine structure of the ineffective nodules produced by TL16 on Cicer arietinum L changed in comparison with those produced by the wild type. The mutant strain TL16 suffered a disruption in the rpoN gene. Only one copy of rpoN gene is present in M. ciceri. The mutation abolishes Dct activity. It additionally abolishes the symbiotic nitrogen fixation activity of the bacteroids in the nodules. This first document in M. ciceri shows that a functional rpoN gene is essential for the transport of dicarboxylic acids and symbiotic nitrogen fixation.