1. Field acoustic playback experiments were conducted with maleEleutherodactylus coqui andE. portoricensis. Periodic tone bursts of intensities similar to natural sounds in the habitat of the frogs were used to create sonic interference. The period between tone bursts, the ‘time window’, was varied in duration and in intensity relative to the tone burst. 2. Males of both species suppressed vocalizations during the stimulus tone bursts. The amount of suppression decreased as the tone burst was lengthened. 3. Males of bothE. coqui andE. portoricensis suppressed calling in response to tone bursts of 0.40 to 2.0 kHz, a range which encompasses the principal frequency components present in their vocalizations. 4. BothE. coqui andE. portoricensis initiated significantly (P<0.01) more calls within the time windows between stimulus tone bursts than would be expected by chance when the window duration was as short as 0.25 s and 0.10 s, respectively. These durations are approximately 10% of the spontaneous call repetition periods for each species. 5. E. coqui initiated significantly (P<0.01) more calls than would be expected by chance in tone-filled time ‘windows’ which were only 4 to 6 dB less intense than the tone bursts. This ability of intensity discrimination under sonically adverse natural field conditions indicates a level of performance in the same range as that of mammals. 6. The ability of frogs to call preferentially during very brief silent periods or during periods of slight intensity reductions is viewed as an adaptation for avoiding acoustic interference, and thus improves the efficiency of acoustic communication in an intense and complex sonic environment.