Bromthymol blue, at a concentration of 0.1% in tryptose-glucose broth, inhibited growth of 98.4% of Escherichia coli serotype O157:H7 isolates but only 0.8% of E. coli non-O157:H7 isolates after an overnight incubation at 44.5 degrees C, but not 35 degrees C. The inhibition was dependent on temperature, density of inoculum, bromthymol blue concentration, time of incubation, and composition of the medium. Compared with serologic typing, the inhibition had sensitivity, specificity, predictive values of the positive and negative tests, and overall agreement between the two tests of 98.4, 99.2, 98.4, 99.2, and 98.9%, respectively. The inhibition could be useful as a presumptive test to identify E. coli isolates of serotype O157:H7, especially in laboratories that do not have serotyping capabilities.