Analysis of the DNA sequence of a 17 kb region of the coli lambdoid phage H-19B genome located the genes encoding shiga-like toxin I (Stx-I) downstream of the gene encoding the analogue of the phage lambda Q transcription activator with its site of action, qut at the associated pR' late promoter, and upstream of the analogues of lambda genes encoding lysis functions. Functional studies, including measurement of the effect of H-19B Q action on levels of Stx expressed from an H-19B prophage, show that the H-19B Q acts as a transcription activator with its associated pR'(qut) by promoting readthrough of transcription terminators. Another toxin-producing phage, 933W, has the identical Q gene and pR'(qut) upstream of the stx-II genes. The H-19B Q also activates Stx-II expression from a 933W prophage. An ORF in H-19B corresponding to the holin lysis genes of other lambdoid phages differs by having only one instead of the usual two closely spaced translation initiation signals that are thought to contribute to the time of lysis. These observations suggest that stx-I expression can be enhanced by transcription from pR' as well as a model for toxin release through cell lysis mediated by action of phage-encoded lysis functions. Functional studies show that open reading frames (ORFs) and sites in H-19B that resemble components of the N transcription antitermination systems controlling early operons of other lambdoid phages similarly promote antitermination. However, this N-like system differs significantly from those of other lambdoid phages.