A site within the phage lambda Q gene shares homology with the CII-activated promoters, pE and pI, and is oriented in the direction opposite to that of Q gene transcription. A DNA fragment containing this site can serve as a template for CII-activated transcription in vitro. To ask if this presumptive CII control site functions as a CII-activated promoter in vivo, a restriction fragment containing this promoter has been cloned on a plasmid so that synthesis of beta-galactosidase will be under its control. When CII protein is supplied in trans from a compatible plasmid, this promoter, designated PaQ, is activated to produce beta-galactosidase. A promoter positioned within the Q gene which can be activated by CII protein to initiate transcription in an anti-sense direction should result in an interference with Q gene expression, enhancing CII regulation of late functions, and adding to the list of known CII controls on the lysogenic response.