Male bullfrogs emit multicroak, quasiharmonic advertisement calls that function in mate attraction and neighbor recognition. The degree of variability of acoustic features in these calls can influence perceptual decisions by conspecific receivers. Analysis of duration of individual croaks in spontaneous advertisement calls of a sample of males shows considerable intraindividual variability in this feature, even within short chorusing bouts. The influence of this intraindividual variability on behavior was examined in a series of evoked calling experiments. When presented with synthetic calls whose croak durations varied over the range of the natural variability in this feature, males responded similarly to intermediate and long duration croaks, but significantly less to short duration croaks. When presented with playbacks of calls with croak durations outside the natural range of variability, males again responded significantly less to shorter durations. The response gradient for duration is thus asymmetrical, with stimuli at the shorter end of the continuum evoking fewer responses than stimuli at the longer end. This asymmetry may be related to the biological demands of rejecting perception of heterospecific advertisement calls, and of mediating appropriate responses to conspecific aggressive calls. The shape of the response gradient for duration may reflect a process of stimulus generalization.