Response properties of the auditory nerve fibers of the Pacific treefrog, Hyla regilla, were shown to be comparable to other Hylids at a standard temperature of 20 degrees C. This species from a rather variable thermal habitat was compared to the neotropical aboreal species, Eleutherodactylus coqui, in a study of auditory responses under conditions of changing temperatures. We found that as temperature increased: the center frequencies (CFs) of tuning curves remained constant for neurons from the basilar papilla but shifted to higher frequencies for neurons from the amphibian papilla (the amount of frequency shift decreased as CF increased); tone response thresholds decreased; phase-locking became stronger; the phase of the preferred response showed a lag in neurons with CFs below 500 Hz, a lead for CFs above 600 Hz, and almost no changes for CFs between 500 and 600 Hz; the variance of the click response and the average latency of the first spike in the response decreased. Threshold and vector strength measurements suggest a thermal optimum of about 20-22 degrees C in H. regilla whereas this optimum in E. coqui seems to lie at 25 degrees C or higher.