We have identified and characterized in the supernatant of the transition phase of Bacillus thuringiensis var. kurstaki the peptide SKPDT. This peptide was previously identified by in silico analysis by Pottathil and Lazazzera (Front Biosci 8:32-45 2003) as a putative signaling peptide (NprRB) of the Phr family in B. thuringiensis. The chemically synthesized NprRB did not affect the growth kinetics of B. thuringiensis var. kurstaki but stimulated the sporulation, spore release, and transcription of crylAa when added to cultures during the transition phase. In fact, when the peptide (100 nM) was added to a culture in transition phase, the transcription of crylAa was stimulated almost threefold, mainly from the late promoter BtII, which requires the late-stage sporulation-specific transcription factor sigma(k). On the other hand, NprRB did not have any effect on B. subtilis. Thus, SKPDT seems to be a signaling peptide specific for B. thuringiensis.