E. coli lysogenic for the temperate, lambda-related phage HK022 do not support lambda growth. The exclusion of lambda is caused by the HK022 nun gene product, which blocks the expression of genes located downstream of and in the same transcription unit as the lambda nutL and nutR sequences. Transcripts terminating prematurely at or near nutR have been detected after inactivation of lambda repressor in lambda, HK022 dilysogens. Nun therefore appears to be a transcription termination factor with a remarkable specificity; it converts the lambda nut sequences, which normally interact with lambda N protein to suppress transcription termination, into terminators. These and other similarities between Nun-promoted termination and N-promoted antitermination argue strongly that the mechanisms of the two reactions have steps in common.