In the present study, we examined the frequency dependence of auditory search performance. Detection thresholds were measured for an 800-Hz target tone in a sequence of distractor tones (informational masking) as a function of frequency separation between the target and the distractor tones. The results showed that the thresholds decreased monotonically with frequency separation increasing. To further quantify the frequency dependence of auditory search performance, we applied a roex function (Patterson and Moore, 1986) to estimate a filter bandwidth for the threshold data. The estimated bandwidth was wider than that of an auditory filter by a factor of five (Leek et al., 1991). This result, together with some earlier results (Mori, 2003), demonstrates the effectiveness of the informational masking we used.