The cbnA gene encoding chlorocatechol dioxygenase from the soil bacterium Ralstonia eutropha NH9 under the control of a modified cauliflower mosaic virus 35S promoter was introduced into a hybrid poplar (Populus tremula x P. tremuloides). Integration of the cbnA gene in transgenic poplar was confirmed by PCR and genomic Southern blot analysis. Expression of the cbnA gene was analyzed by Western blot analysis. Transgenic poplar calli efficiently converted 3-chlorocatechol to 2-chloro-cis,cis-muconate.