In Clostridium acetobutylicum, abrB310 is transcribed from two transcription start sites (designated A1 and A2) forming an abundant large, and a five- to tenfold less abundant small transcript, respectively throughout exponential, acidogenic growth and early in the transitional period to stationary, solventogenic growth. beta-galactosidase reporter vectors were constructed to compare the transcriptional activity of the entire abrB310 promoter and the A1 and A2 transcription start sites individually. In stark contrast to the primer extension data, the A2 start site was threefold more active than the entire promoter, which was threefold more active than the A1 start site in wild type C. acetobutylicum. The activity expressed from all three reporter vectors declined as the cultures transitioned from exponential to stationary growth. In the spo0A-deficient strain SKO1, reporter vector activity continued for 10 h into stationary growth. The removal of the putative Spo0A binding site from all three vectors had no significant effect on promoter activity in either wild type or SKO1. We conclude that the presence of both the A1 and A2 transcription start sites is required for the correct control of abrB310 expression, and that AbrB310 is necessary but not sufficient for the correct transition between acidogenic and solventogenic growth.