Bacterial strains of the species Arthrobacter nicotianae, Corynebacterium ammoniagenes, Corynebacterium casei, Microbacterium barkeri, Microbacterium gubbeenense and Staphylococcus equorum subsp. linens, all isolated from the smear of surface ripened cheeses, were found to possess autoinducer-2 (AI-2) activity using the Vibrio harveyi (BB170) bioluminescence assay. In contrast, Brevibacterium casei and Brevibacterium linens strains were not found to have AI-2 activity. When exposed to low pH and high NaCl concentrations, AI-2 activities increased between 5.0 and 11.6× for C. casei 44701, M. gubbeenense 15944 and S. equorum subsp. linens 15097, whereas AI-2 activities did not increase for A. nicotianae 20123, C. ammoniagenes 20305, C. ammoniagenes 20306 and M. barkeri 20145. The fact that AI-2 activity is produced by bacterial strains found in the smear of surface ripened cheeses, and also increased by dairy-relevant stress conditions, indicates that AI-2 signalling might be important in regulation of microbial succession during ripening of surface ripened cheeses.