Escherichia coli O157:H7 strains 933 produces elevated levels of 2 phage-encoded, antigenically distinct cytotoxins designated Shiga-like toxin I (SLT-I) and Shiga-like toxin II (SLT-II). These toxins kill both HeLa and Vero cells. In this report, the relationship between SLTs and a cytotoxin produced by E. coli strains isolated from pigs with edema disease (ED) was examined. Culture filtrates from 72 out of 81 ED strains were cytotoxic for Vero but not HeLa cells. Cytotoxicity was neutralized by antiserum to SLT-II but not by anti-Shiga toxin. No toxin-converting phage were detected in 20 toxigenic ED strains examined. The cytotoxin of the ED-causing strains appears to be a variant of SLT-II and production of this cytotoxin is not phage-mediated.