In order to compare molecular structure, malonate decarboxylases from Acinetobacter calcoaceticus, Pseudomonas fluorescens, and Pseudomonas putida aerobically grown on malonate, were purified by the method employing streptomycin sulfate treatment, chromatography with PBE 94 and ${omega}-aminohexyl$ agarose. Molecular masses were estimated to be 185, 200, and 200 kDa, respectively. All malonate decarboxylases were multimeric enzymes consisting of four different subunits, $2{alpha},;1{eta},;1{gamma},;and;1{delta}$. The molecular masses of the Pseudomonas enzyme subunits were $65({alpha})$, $33({eta})$, $30({gamma})$, and $11kDa({delta})$; which are very similar to those, $65({alpha})$, $32({eta})$, $25({gamma})$, and $11kDa({delta})$ of Acinetobacter enzyme. The ${delta}-subunit$ of the active form of the enzymes was acetylated. The acetyl group may form a thioester bond with the thiol group of the prosthetic group covalently linked to the enzyme. It suggests that such molecular organization is common in all malonate decarboxylases.