An oxaloacetate (OAA) decarboxylase (EC 4.1.1.3) has been purified 72-fold from Acetobacter aceti cells grown on ethanol, and its molecular weight was estimated to be about 80,000 by gel filtration. Several properties distinguished this enzyme from the OAA decarboxylase from A. xylinum: a) It was not a constitutive enzyme; the activity was 6- to 20-fold higher in cells grown on a C2 substrate (acetate or ethanol) than in cells grown on a C3 compound (pyruvate or propionate). b) The optimum pH was 7.5; a value of 5.6 was reported for the enzyme from A. xylinum. c) The enzyme did not need a divalent cation and was not inhibited by EDTA. d) The Kmvalue for OAA was found to be 0.22 mM. It was not affected by the addition of nicotinamide adenine dinucleotide. e) The enzyme activity was neither inhibited by acetate nor by L-malate.