Interaural amplitude jnd's were measured for conditions in which a pure tone of a fixed frequency, f0, was presented to one ear and a tone of frequency f0+Δf to the other. Interaural amplitude sensitivity was found to decrease as Δf was increased. This result suggests that the binaural interaction takes place within a limited frequency region. It was also found that the frequency region for which a binaural interaction is possible increased with signal frequency in a manner reminiscent of monaural critical-band findings. Finally, the range of Δf over which the task could be performed was considerably larger than that usually reported for binaural fusion of pure tones. For example, for f0=3000 Hz, an interaural amplitude jnd of less than 5 dB was obtained for Δf=1000 Hz.