One of the main virulence factors produced by Shiga toxin-producing Escherichia coli is the Shiga toxin (Stx), which is encoded on lambdoid phages (Stx phage). In Norway, an outbreak of hemorrhagic colitis and hemolytic uremic syndrome (HUS) caused by E. coli O103:H25 was reported during the winter of 2006, but stx(2)-positive isolates were only retrieved from two human samples.Isolates of E. coli O103:H25 from patients with HUS in Norway, including sporadic cases and the outbreak cases, were investigated for the presence of phages encoding stx(2). The induced Stx phages were characterized morphologically and genetically, and the host susceptibility for these phages of various E. coli O103 isolates, including O103:H25 stx(2) negative isolates from the outbreak, was tested by a plaque assay.The Stx2 phages in this study are very closely related in terms of morphology, sequence identity, and host infectivity. There may be a conserved phage within the E. coli O103:H25 population.It is proposed that the Stx2 phage, present in the environment either as free phage particles or within a limited pool of Stx-producing E. coli O103 strains, have infected or integrated in the stx(2)-negative E. coli O103:H25 isolates from the Norwegian outbreak.