The bacterial strains of the genus Nocardia were used for the bioconversion of fumaric acid to l-malic acid. The ability of the bacterial strain Nocardia sp. CCM 4837/A to produce l-malic acid from fumaric acid was investigated under various conditions. The optimal temperature for the bioconversion was approximately 37 A degrees C, and the optimal pH was around 8.0. The addition of an inductor (fumarate salt) to the fermentation medium was necessary to enhance enzyme activity. The presence of detergent Triton X-100 (0.02-0.1 %) in the reaction mixture rapidly increased the enzyme activity of fumarase. The specific fumarase activity of intact cells Nocardia sp. CCM 4837/A increased from 2.8 to 75 U/mg after optimising the experimental conditions described here. Pretreatment of the Nocardia cells with malonate was not necessary because succinate was not detected as a by-product under our experimental conditions.