Transcriptional studies of gene expression in coliphage λ reveal a complex set of interactions involving host RNA polymerase and the positive and negative regulatory functions of the phage genes N and cro, respectively. The N product, which is known for its anti-terminating effect, was also found to stimulate up to 4-fold the rate of transcription in the pL-N region, as determined by comparing induced λN+cro− and λN−cro− lysogens. In contrast, the Tof function of the cro gene product rapidly turns off transcription in the same region shortly after thermal induction of λN+cro+ lysogens. In λN−cro+ lysogens, however, this early turn-off was not observed, suggesting that the effective level of Tof function is reached only under N+ conditions. This N requirement for the early expression of Tof function in λ was also observed in induced prophages lacking all phage genes except those in the N-cI-cII region and, in addition, was found not to be satisfied by the analogous N product from phage λimm21. Furthermore, it was found that neither the phage mutation nutL− nor the host mutation nusA− blocks the effect of N protein on Tof function, indicating that antitermination per se is not required for Tof expression. However, since the expression of Tof function is blocked in the groN− host, which has an alteration in the β subunit of RNA polymerase, an interaction involving λ N protein and the host RNA polymerase might be required for Tof expression. Although these results are in apparent contradiction with the Fed phenomenon, we do find at late times after induction, when comparing N-cro+ N−cro− lysogens, that the rate of transcription under cro− conditions is up to 2-fold higher in the pL-proximal region and up to 10-fold higher in the exo-gam region. This indicates that the residual read-through transcription of red-gam genes under N− conditions is controlled by cro. One may conclude that, although the cro product may function through an operator-repressor type of interaction at the promoter-operator regions, its action at pL and pR may be augmented by more complex interactions involving Escherichia coli RNA polymerase and the λ regulatory protein N.