The anti-terminator Q933 gene of the bacteriophage 933W was evaluated as a marker for Escherichia coli O157:H7 strains with high Shiga toxin production. In total, 262 environmental strains of E. coli O157:H7 isolated from feces of beef cattle and the digestive tract of houseflies were screened for the Q933 and Q21 (anti-terminator Q21 of bacteriophage 21) genes by polymerase chain reaction. Nine (3.4%) isolates tested positive for Q933 alone, 161 (61.5%) were positive for the Q21 gene alone, and 92 (35.1%) isolates carried both Q alleles. Results from the enzyme-linked immunosorbent assay show that the isolates with Q933 alone produced significantly more Shiga toxin than the remaining isolates. The difference was even greater after the induction of the toxin production by a short exposure of cells to ultraviolet light. These data suggest that Q933 is a promising indicator for environmental E. coli O157:H7 with high production of Shiga toxins and, therefore, for potentially clinically relevant strains.