The strictly anaerobic bacterium Clostridium acetobutylicum is well known for its ability to convert sugars into acids and solvents, most notably the potential biofuel butanol. However, the regulation of its fermentation metabolism, in particular the shift from acid to solvent production, remains poorly understood. The aim of this study was to investigate whether cell-cell communication plays a role in controlling the timing of this shift or the extent of solvent formation. Analysis of the available C. acetobutylicum genome sequences revealed the presence of eight putative RNPP-type quorum sensing systems, here designated qssA to qssH, each consisting of RNPP-type regulator gene followed by a small open reading frame encoding a putative signalling peptide precursor. The identified regulator and signal peptide precursor genes were designated qsrA to qsrH and qspA to qspH, respectively. Triplicate mutants were generated for each system in C. acetobutylicum ATCC 824 and screened for phenotypic changes. Endospores counts after 7 days were only affected in the qsrG-deficient strains, whereas solvent formation was affected in all mutants with the exception of the qsrD-deficient strains. The qsrB mutants were of particular interest as they showed increased solvent formation during early solventogenesis. Overexpression of qsrB considerably reduced solvent and endospore formation, suggesting that it acts as a repressor. Addition of short synthetic peptides representing internal fragments of QspB counteracted QsrB-mediated repression and restored both solvent production and sporulation. Together, these findings support the hypothesis that QssB is a functional quorum sensing system involved in the regulation of early solventogenesis and sporulation.