Selective attention has been hypothesized to reduce distractor interference at both perceptual and postperceptual levels (Lavie, 2005), respectively, by focusing perceptual resources on the attended location and by blocking at postperceptual levels distractors that survive perceptual selection. This study measured the impact of load on these selection mechanisms using a flanker paradigm (Eriksen & St. James, 1986) and indexing distractor interference as a function of separation. It distinguished changes in the "extent of focus" of the distractor-interference function of separation (reflecting perceptual selection) from changes in the amplitude of distractor interference "not accompanied" by changes in focus (reflecting postperceptual selection). It showed that: (1) the spatial profile of perceptual resources is shaped like a "Mexican hat" (Muller et al., 2005); (2) increasing perceptual load "focuses" perceptual resources (Caparos & Linnell, 2009); (3) increasing cognitive load "defocuses" perceptual resources; and (4) participants with reduced working-memory span show reduced postperceptual blocking of distractors. While these findings are consistent with two levels of selective attention, they show that the first perceptual level is affected not only by perceptual but also by cognitive-control mechanisms. (Contains 16 figures and 7 footnotes.)