This work reports the biochemical and functional analysis of the J2315 gene, encoding a protein with GDP-D-mannose 4,6-dehydratase enzyme activity (E.C.4.2.1.47). Data presented indicate that the protein is active when in the tetrameric form, catalyzing the conversion of GDP-D-mannose into GDP-4-keto-6-deoxy-D-mannose. This sugar nucleotide is the intermediary necessary for the biosynthesis of GDP-D-rhamnose, one of the sugar residues of cepacian, the major exopolysaccharide produced by environmental and human, animal and plant pathogenic isolates of the complex species. V and K values of 1.5±0.2 µmol.min.mg and 1024±123 µM, respectively, were obtained from the kinetic characterization of the J2315 BceN protein by NMR spectroscopy, at 25°C and in the presence of 1 mol MgCl per mol of protein. The enzyme activity was strongly inhibited by the substrate, with an estimated K of 2913±350 µM. The lack of a functional gene in a mutant derived from IST408 slightly reduced cepacian production. However, in the ATCC17616 with as the single gene in its genome with predicted GMD activity, a mutant did not produce cepacian, indicating that this gene product is required for cepacian biosynthesis.