Competence for genetic transformation in the human pathogen Streptococcus pneumoniae is a transient physiological property. A competence-stimulating peptide, CSP, was recently identified as the processed product of the comC gene. As conflicting results have been reported regarding CSP autoinduction, we monitored the CSP-induced expression of comCDE in derivatives of strain R6 using comC::lacZ fusions. Autoinduction was demonstrated in this genetic background. The kinetics of CSP-induced transcription of comCDE and of a late competence-induced (cin) operon were compared. While the comCDE mRNA level was highest 5 min after CSP addition then decreased, maximal cin expression required 10 min exposure to CSP. Transformation frequencies paralleled cin expression. After 20 min exposure to CSP, both mRNAs disappeared almost completely, providing evidence for an intrinsic mechanism for shutting off CSP signal transduction. Investigation of spontaneous competence development in mixed cultures indicated that transformation of wild-type cells was delayed in the presence of CSP non-producers, consistent with a direct role of CSP in quorum sensing. The effect of varying inoculum size on the timing of competence development was investigated. While competence developed in wild-type cultures at a similar critical density, about OD550 = 0.15, a mutant lacking the three oligopeptide-binding lipoproteins transformed at a 50-fold reduced cell density. The latter effect was mimicked in a strain harbouring a duplication of comC. Altogether, these results suggest that CSP does not accumulate passively in pneumoccal cultures, but that comCDE basal expression can be modulated.