In order to assess the binaural system's ability to detect varying interaural temporal differences (IATD's) we required observers to discriminate a binaural noise with a sinusoidally modulated IATD (a “moving dot”) from a binaural noise with random static IATD's (a “stationary line”). This latter stimulus was constructed so that its image presumably filled the same intracranial space as that described by the excursions of the “dot.” For several modulation frequencies (fm) from 0 to 200 Hz we determined the peak IATD (Δtmax, corresponding to the extent of movement) required for 75% discriminability of the dot from the line. Asfmincreased from 0 to 50 Hz, threshold Δtmaxincreased from 30 to 130 μsec, reflecting the decreasing ability of the binaural system to follow the fluctuating IATD's (forfm>50‐Hz thresholds decreased again, probably because cues other than movement were available at these high rates of modulation). A simple extension of the Jeffress coincidence network model for localization is proposed as a possible “movement detector.”