N-Acylhomoserine lactones (AHLs) are used as quorum-sensing signal molecules by many Gram-negative bacteria. We have reported that Microbacterium testaceum StLB037, which was isolated from the leaf surface of potato, has AHL-degrading activity. In this study, we cloned the aiiM gene from the genomic library of StLB037, which has AHL-degrading activity and shows high homology with the ␣/␤ hydrolase fold family from Actinobacteria. Purified AiiM as a maltose binding fusion protein showed high degrading activity of AHLs with both short-and long-chain AHLs with or without substitution at carbon 3. High-performance liquid chromatography analysis revealed that AiiM works as an AHL lactonase that catalyzes AHL ring opening by hydrolyzing lactones. In addition, expression of AiiM in the plant pathogen Pectobacterium carotovorum subsp. carotovorum reduced pectinase activity markedly and attenuated soft rot symptoms on potato slices. In conclusion, this study indicated that AiiM might be effective in quenching quorum sensing of P. carotovorum subsp. carotovorum.