Onset dominance in sound localization was examined by estimating observer weighting of interaural delays for each click of a train of high-frequency filtered clicks. The interaural delay of each click was a normal deviate that was sampled independently on each trial of a single-interval design. In Experiment 1, observer weights were derived for trains of n = 2, 4, 8, or 16 clicks as a function of interclick interval (ICI = 1.8, 3.0, or 12.0 msec). For small n and short ICI (1.8 msec), the ratio of onset weight to remaining weights was as large as 10. As ICI increased, the relative onset weight was reduced. For large n and all ICIs, the ongoing train was weighted more heavily than the onset. This diminishing relative onset weight with increasing ICI and n is consistent with optimum distribution of weights among components. Efficiency of weight distribution is near ideal when ICI = 12 msec and n = 2 and very poor for shorter ICIs and larger ns. Further experiments showed that: (1) onset dominance involves both within- and between-frequency-channel mechanisms, and (2) the stimulus configuration (ICI, n, frequency content, and temporal gaps) affects weighting functions in a complex way not explained by cross-correlation analysis or contralateral inhibition (Lindemann, 1986a, 1986b).