A brief diotic conditioner has been shown to effectively disrupt lateralization of a brief dichotic probe presented after a short interval (4–10 ms, onset to onset) with an interaural time delay that is clearly discriminable in the absence of the conditioner [e.g., P. M. Zurek, J. Acoust. Soc. Am. 66, 1750–1757 (1980)], even when the conditioner and the probe are different sounds [P. L. Divenyi and J. Blauert, in Auditory Processing of Complex Sounds, edited by W. A. Yost and C. S. Watson (Erlbaum, Hillsdale, NJ, 1987), pp. 146–155]. The present experiments investigated lateralization suppression of a narrow-band dichotic probe centered at 2 kHz by a narrow-band diotic conditioner, in situations in which the probe could not be regarded as an echo of the conditioner. In one case, the conditioner was identical to the probe but was presented in opposite phase; in other conditions, the center frequency of the conditioner was different, with the center frequency to bandwidth ratio remaining constant. The effects of frequency and temporal separation between the conditioner and the probe on the suppression of the lateralization of the probe were explored. Suppression of lateralization was measured as the increase in the just-noticeable interaural time difference (jnd), with respect to the interaural time jnd obtained for the probe alone. Low-frequency (0.5 to 1.5 kHz) conditioners were more effective in suppressing the lateralization of the probe than those at or above the probe frequency. The results suggest that a diotic stimulus with a relatively high localization strength (i.e., one that yields low interaural time jnd’s when presented in isolation) could ‘‘mask’’ the localization of a subsequent dichotic stimulus having a lower localization strength.