Woodhouse's toad (Bufo woodhousei), in contrast to many other anurans, has one all-purpose call used by the male to attract females as well as to announce his presence to competing males. The call has a pulse rate of about 160 per second and a dominant frequency of about 1500 Hz. The rate of calling is specific to an individual and may range from one to eight calls per minute. Although male call rate is correlated with mating success, males reduce their call rate in response to playbacks of conspecific calls [B. K. Sullivan, J. Herpetol. (in press)]. By observing the alteration of call rates of males in response to a series of synthesized calls, we hoped to identify some important acoustic characteristics of the natural call. The synthetic calls were either complex tones or bandpassed noise, with systematic variation of the dominant frequency or the pulse rate. The calls were presented over a small speaker to individual male toads in their natural environment. The toads responded equally well to synthetic calls with different pulse rates if the dominant frequency was preserved. Maintaining the typical pulse rate while changing the dominant frequency, however, was less effective in eliciting changes in calling rate. Changes in call rate in response to a wide range of synthetic calls may reflect the male's attempts to avoid acoustic interference from other species in his environment, not only his own conspecific rivals.