Abstract Shiga toxins (Stx) comprise a family of potent cytotoxins that are involved in severe human disease. Stx are mainly produced by Escherichia coli isolated from human and nonhuman sources, and by Shigella dysenteriae type 1. The genes encoding Stx are thought to be generally encoded in the genome of lambdoid prophages (Stx-converting bacteriophages; Stx phages). They share a unique position in the late region of the phage genome downstream of the late promoter p R′. This location suggests that expression of stx is controlled by a Q-like antiterminator. Therefore, induction of Stx-converting prophages appears to trigger increased production of Stx. Following induction, Stx phages can be transduced in vivo and in vitro into other bacteria. Stx phages play an important role in the expression of Stx and in lateral gene transfer and are therefore a contribution to the emergence of new Stx-producing E. coli (STEC) variants.