Auditory sensitivity in six bullfrog tadpoles was investigated by electrophysiological techniques. Tonal stimulus intensities that generated a standard potential of 0.1 μV in the inner ear showed a hearing range from 100 to 4000 Hz with maximum sensitivity near 1 dyn/cm2 (0.1 N/m2) in the lower portion of the range. The data also indicated that tadpoles lack the place mechanism of adult frogs and thus place as a mechanism appeared at the amphibian metamorphosis in the course of the evolution of vertebrate hearing.