gram-positive aerobic bacterium, Arthrobacter sp. strain TPU 5446 with a high maleate cis-trans isomerase (MAI) activity, was isolated from soil. Enzyme activity was induced when the strain was cultivated in the presence of maleic acid and yeast extract as the carbon and nitrogen sources, respectively. From a crude extract of the cells, the enzyme was partially purified by ammonium sulfate fractionation and DEAE-Toyopearl column chromatography. The enzyme acted specifically on maleic acid and produced fumaric acid stoichiometrically.