Lysogens of phage HK022 are resistant to infection by phage . Lambda resistance is caused by the action of the HK022 Nun protein, which prematurely terminates early  transcripts. We report here that transcription of the nun gene initiates at a constitutive prophage promoter, P_Nun, located just upstream of the protein coding sequence. The 5' end of the transcript was determined by primer extension analysis of RNA isolated from HK022 lysogens or RNA made in vitro by transcribing a template containing the promoter with purified Escherichia coli RNA polymerase. Inactivation of P_Nun by mutation greatly reduced Nun activity and Nun antigen in an HK022 lysogen. However, a low level of residual activity was detected, suggesting that a secondary promoter also contributes to nun expression. We found one possible secondary promoter, P_Nun', just upstream of P_Nun. Neither promoter is likely to increase the expression of other phage genes in a lysogen because their transcripts should be terminated downstream of nun. We estimate that HK022 lysogens in stationary phase contain several hundred molecules of Nun per cell and that cells in exponential phase probably contain fewer.