Presence of the luxS gene that encodes LuxS autoinducer 2 (AI-2) synthase in Streptococcus bovis was demonstrated, and the molecular properties and transcription of the gene were examined. The S. bovis luxS was transcribed in a monocistronic fashion. Intracellular luxS-mRNA increased sharply during the initial exponential growth, and decreased abruptly after the middle exponential phase. The large drop in luxS transcription began before the glucose supply to cells decreased or the growth rate declined. Transcription of luxS was not directly related to cell density, and continued at a maximal rate when cells were kept growing at a maximal rate. It is conceivable that AI-2 activity in S. bovis acts as a signal for adjusting cell physiology and metabolism in response to environmental conditions. However, the role of LuxS in S. bovis, including the regulation of AI-2 synthesis,, remains to be clarified.