Recent archaeological investigations along the lower Negro and upper Xingu Rivers in the Brazilian Amazon provide important new evidence bearing on long-standing debates about the size and permanence of Amerindian settlements in the region. Preliminary regional surveys and more in-depth study of selected large (30-50 ha) sites, particularly analyses of the associations between structural features, anthropogenically altered soils, and artifact distributions, lead us to conclude that large, permanent settlements, likely associated with fairly dense regional populations, existed prehistorically in both areas. These findings cast doubt on the view that environmental limitations prevented sedentism and demographic growth among Amerindian populations throughout much or all of the region. Specifically, we conclude that fully sedentary and relatively large populations emerged in a variety of Amazonian settings prehistorically, not necessarily correlated with the distribution of one or another narrowly defined ecological variable (e. g., high fertility soils). Thus, a critical evaluation of core concepts in Amazonian anthropology, such as the várzea/terra firme dichotomy or tropical forest culture, is advised.