Quorum sensing involving acyl homoserine lactones (AHLs) is a density-dependent form of intercellular communication that occurs in many different members of the group Proteobacteria. However, to date, there have been few investigations of its occurrence in cyanobacteria. Here, using both a bioreporter Agrobacterium tumefaciens NTL4 (PZLR4) and mass spectrometry, we provide evidence of N-octanoyl homoserine lactone (C8-AHL) production in axenic cultures of the cyanobacterium Gloeothece PCC6909 and its sheathless mutant PCC6909/1. Accumulation of C8-AHL in the culture medium of laboratory cultures of Gloeothece followed a pattern characteristic of the phenomenon of autoinduction, a common feature of functional AHL-based quorum-sensing systems. Analysis by two-dimensional gel electrophoresis showed that, in response to treatment with C8-AHL, early growth-stage cells of PCC6909/1 showed changes in expression of 43 proteins compared with untreated cells. Among the 15 proteins that showed more than a twofold change in expression were RuBisCo, glutamate synthase, chorismate synthase, a member of the LysR family of transcriptional regulators (all upregulated), and enolase and aldolase, both of which were downregulated. The significance of such changes in response to C8-AHL is discussed in relation to carbohydrate and amino-acid metabolism and involvement of Gloeothece in biofilms.