Microorganisms which produce fumaric acid from maleic acid were screened. Strain XD-1 was selected as the best strain. Based on its taxonomical characteristics, this strain was concluded to be a Pseudomonas alcaligenes strain. Addition of toluene to the reaction mixture enhanced the bioconversion of maleic acid to fumaric acid. The conversion activity was stable at up to 45 degrees C. Under the optimum conditions, the fumaric acid productivity of the strain reached 41.9 g/l at 6 h of incubation. However, when the incubation time was extended to more than 6 h, the fumaric acid was converted to L-malic acid.