To measure the changes in substrate composition produced by spawning brook troutSalvelinus fontinalis, we collected 187 freeze-core samples, both before and after spawning, from egg pockets, inside redds but excluding egg pockets, and outside redds. In the upper strata of reddassociated samples, we found that the proportions of particles 0.85–1.70, 0.425–0.85, 0.212–0.425, and less than 0.212 mm in diameter were significantly lower in egg pockets than outside redds, and the proportions of particles 0.425–0.85, 0.212–0.425, and less than 0.212 mm in diameter were also lower in egg pockets than in other areas inside redds. Samples collected outside redds were similar to samples collected before spawning. We observed no significant correlations between the proportion of fine sediment in samples from outside redds (and presumably the proportion present before spawning) and the proportion of fine sediment in egg pockets. In vertically stratified samples, the lower strata contained a greater proportion of the smaller substrate particles than the upper strata. Spawning brook trout removed fine sediment from the substrate during redd construction, and this removal was most apparent in the egg pocket. Thus, egg pockets, rather than entire redds, should be the focus of studies of survival to emergence.