1. Noctuid moths of several species were mounted at the tip of a tower of fine tubing in acoustic ‘free space’. Recordings were made of the intensity of a brief pulse of ultrasound necessary to produce a constant tympanic nerve response for any angle of sound presentation relative to the moth's body axis. Such plots of intensity versus angle were made with the wings held in several postures approximating those assumed in normal flight. 2. The data indicate that sound intensity reaching the tympanic organ can vary by as much as 40 db. depending upon: ( a ) the position of a sound source relative to the moth's body axis, and ( b ) the position of its wings. 3. With wings above the horizontal plane each ear reports sounds c . 20-40 db. louder on the ipsilateral side than on the contralateral side. With wings below the horizontal, the lateral asymmetries are replaced by a dorsoventral asymmetry in which each ear reports sounds coming from below the body c . 10-25 db. louder than sounds coming from above. 4. Directional sensitivity plots at 60 kcyc./sec. are more complex than plots at 30 kcyc./sec.--as expected. 5. A theory is presented to explain how a moth could determine the direction of a sound source in three-dimensional space by comparing the intensity reports of both tympanic organs during a complete wing cycle.