Through the present study, the acoustic information available to an echolocating bat that uses brief frequency-modulated (FM) sonar sounds for the pursuit and capture of insect prey has been characterized. Computer-generated sonar pulses were broadcast at tethered insects, and the returning echoes were recorded on analog tape at high speed for off-line analyses. Echoes from stationary and fluttering insects were displayed using time waveform, spectrogram, power spectrum, and cross-correlation representations. The results show echo signatures for the different insect species studied, which change with the angle of incident sound. Sequences of echoes from fluttering insects show irregular changes in sound amplitude and time-frequency structure, reflecting a random temporal relation between the changing wing position and the arrival of incident sound. A set of recordings that controlled the temporal relation between incident sound and insect wing position suggests that information about the spatial profile of a flying insect could be enhanced if the bat were to produce a sequence of sounds that synchronized briefly with the moving target's wing-beat cycle. From this study, it has been proposed that the FM bat receives stroboscopic-like glimpses of fluttering prey whose spatial representation depends on the operation of the bat's sonar receiver.