Male Puerto Rican treefrogs, Eleutherodactylus coqui, produce a two-note call: a 100-msec constant frequency 'Co' note, followed by a longer, upward sweeping 'Qui' note. Previous behavioral studies have shown that males respond selectively to natural and synthetic call notes of 100 msec duration, whereas preliminary results suggest that females respond preferentially to the second note in the male's call. In the present study, we first show that the basilar papilla in the inner ear is tuned differentially in males and females. Comparisons were next made between cells in the eighth nerve and midbrain torus semicircularis of firing rate vs. duration functions in order to help determine the underlying neural mechanisms responsible for the behavioral selectivity to notes of 100 msec duration. A model for detection of vocalizations of specific durations is postulated and discussed in the light of the observed calling behavior of the male as well as the response properties of a class of cells found in the torus semicircularis.