1. The mustached bat, Pteronotus parnellii parnellii, emits bisonar pulses each consisting of eight components: CF1-4 and FM1-4. In the auditory cortex of the bat there are arrays of FM-FM neurons that are tuned to particular delays of echo FMn (n = 2, 3, or 4) from pulse FM1. They are specialized for the processing of target-range information. The FM signal is suited for ranging and also for target localization. Therefore we studied the directional sensitivity of FM-FM neurons with pulse FM1 and echo FMn. One of the FM1-FMn pair was moved around the bat's head while the other was fixed in front of the bat. 2. FM-FM neurons are sharply tuned in echo delay and are broadly tuned in echo amplitude. That is, they are tuned to a target that has a particular cross-sectional area and that is located at a particular distance from the bat. Their best amplitudes for echoes range between 8 and 73 dB sound pressure level (SPL). The best amplitude is approximately 30 dB higher than minimum threshold in the majority of neurons. 3. The higher the best amplitude is relative to minimum threshold, the larger the receptive field is at the best amplitude. The receptive field of FM-FM neurons at 30 dB above minimum threshold is always so large that it covers the entire contralateral auditory field or the entire contralateral field and the medial half of the ipsilateral auditory field. The large size of the receptive field and the uniform distribution of response magnitudes within the receptive field indicate that FM-FM neurons are not suited for sound localization. Directional information is probably processed in parallel by a separate population of neurons other than FM-FM neurons. 4. The receptive field of FM-FM neurons at 10 dB above minimum threshold is much smaller than that at 30 dB above minimum threshold, but it is still large. The mean azimuthal and elevational widths for echo FMn are greater than 70 degrees in all directions. There is no sign that FM-FM neurons are more directional than peripheral neurons. Furthermore, there is neither an azimuthal nor an elevational axis within the FM-FM area. 5. Mean best azimuths of FM-FM neurons are different for each echo FM harmonic: lateral 35 degrees for FM2 and lateral 19 degrees for FM3 and FM4.(ABSTRACT TRUNCATED AT 400 WORDS)