Shiga toxin 2 (Stx2), one of the principal virulence factors of enterohemorrhagic Escherichia coli, is encoded by 933W, a lambda-like prophage. 933W prophage induction contributes to Stx2 production, and here, we provide evidence that Dam methyltransferase is essential for maintenance of 933W lysogeny. Our findings are consistent with the idea that the 933W prophage has a relatively low threshold for induction, which may promote Stx2 production during infection.