We have isolated, purified and characterized arylmalonate decarboxylase (AMDase; EC 4.1.1.76). This is an unique enzyme that gives optically pure arylpropionates from the corresponding arylmalonates. Recently, we have screened similar enzyme producers from soil samples and succeeded in isolating Achromobacter sp. KU1311. The gene encoding the enzyme was cloned and sequenced. The AMDase gene consists of 720 nucleotides, which specifies a 240 amino acid protein with a relative molecular mass of 24,735. This enzyme was purified and its characteristics were compared with those of the hitherto known enzyme from Alcaligenes bronchisepticus KU1201.