Sound pressure of tones of 4, 10, and 20 kHz was measured at the external meatus as a function of the azimuth of the source. The measurements were made under free-field conditions in man, squirrel monkey, bat, and rat. At 4 kHz, rat and man showed consistent variations of sound pressure as a function of azimuth. Interaural intensity differences were estimated to be sufficient to support localization of a tone at a minimum angle of 2.1° (rat) and 1.13° (man) from the midline. No regular sound pressure differences were obtained in bat and squirrel monkey. At 10 and 20 kHz, rat, bat, and monkey showed consistent variations of sound-pressure level with azimuth of at least 15 dB. Interaural differences were sufficient to support localization of a 10-kHz tone at 1.13° (monkey), 2.0° (bat), and 1.5° (rat). Similar results were obtained at 20 kHz. No measurements were made on man at 10 and 20 kHz. [Supported by the National Science Foundation.]