Quorum sensing is a signalling mechanism that controls diverse biological functions, including virulence, via N-acylhomoserine lactone (AHL) signal molecules in Gram-negative bacteria. With the aim of isolating strains or enzymes capable of blocking quorum sensing by inactivating AHL, bacteria were screened for AHL degradation by their ability to utilize N-3-oxohexanoyl-L-homoserine lactone (OHHL) as the sole carbon source. Among four isolates, strain IBN110, identified as Arthrobacter sp., was found to grow rapidly on OHHL, and to degrade various AHLs with different lengths and acyl side-chain substitutions. Co-culture of Arthrobacter sp. IBN110 and the plant pathogen Erwinia carotovora significantly reduced both the AHL amount and pectate lyase activity in co-culture medium, suggesting the possibility of applying Arthrobacter sp. IBN110 in the control of AHL-producing pathogenic bacteria. The ahlD gene from Arthrobacter sp. IBN110 encoding the enzyme catalysing AHL degradation was cloned, and found to encode a protein of 273 amino acids. A mass spectrometry analysis showed that AhlD probably hydrolyses the lactone ring of N-3-hexanoyl-L-homoserine lactone, indicating that AhlD is an N-acylhomoserine lactonase (AHLase). A comparison of AhlD with other known AHL-degrading enzymes, Bacillus sp. 240B1 AiiA, a Bacillus thuringiensis subsp. kyushuensis AiiA homologue and Agrobacterium tumefaciens AttM, revealed 25, 26 and 21 % overall identities, respectively, in the deduced amino acid sequences. Although these identities were relatively low, the HXDH approximately H approximately D motif was conserved in all the AHLases, suggesting that this motif is essential for AHLase activity. From a genome database search based on the conserved motif, putative AhlD-like lactonase genes were found in several other bacteria, and AHL-degrading activities were observed in Klebsiella pneumoniae and Bacillus stearothermophilus. Furthermore, it was verified that ahlK, an ahlD homologue, encodes an AHL-degrading enzyme in K. pneumoniae. Accordingly, the current results suggest the possibility that AhlD-like AHLases could exist in many other micro-organisms.