Citraconate-utilizing bacteria were found to produce d-malate and d-citramalate from maleate and citraconate, respectively. A newly isolated strain DMDC12 showed very stable and high hydratase activity on maleate and citraconate. The strain DMDC12 was identified taxonomically as Arthrobacterpascens. The optical purities of the malate and citramalate produced by this strain were 97.2% and 99.9% d type, respectively. Under optimized conditions, 204.1 g/l of malate and 187.2 g/l of citramalate were produced from maleate and citraconate in 48 h using permeabilized resting cells, with the molar yields of 94.9% and 95.6%, respectively. The hydratase in A. pascens was very stable when 0.6 M of maleate or citraconate was added as the substrate. It was also very stable in the presence of 1 M NaCl. The hydratase was considered to be citraconase since it was strongly induced by citraconate and d-citramalate, but neither by maleate and d-malate nor by 3-hydroxybenzoate and gensitate. These suggested different characteristics between the citraconase of A. pascens and the reported malate-producing enzyme.