Escherichia coli K-12 acquired the ability to produce a high titer of Shiga-like toxin after lysogenization by either of two different bacteriophages isolated from a highly toxinogenic Escherichia coli O157:H7 strain that causes hemorrhagic colitis. One of these phages and another Shiga-like toxin-converting phage from an Escherichia coli O26 isolate associated with infantile diarrhea were closely related in terms of morphology, virion polypeptides, DNA restriction fragments, lysogenic immunity, and heat stability, although a difference in host range was noted. These phages are currently the best-characterized representatives from a broader family of Shiga-like toxin-converting phages.