A large number of newly published and unpublished hectare plots in Amazonia and the Guiana Shield area allow an analysis of family composition andtesting of hypotheses concerning alpha-diversity in the south American rainforest. Using data from 94 plots the family-level floristic patterns in wettropical South America are described. To test diversity patterns, 268 plots are used in this large area. Contrary to a widely held belief, western Amazonian plots are not necessarily the most diverse. Several central Amazonian plots have equal or even higher tree diversity. Annual rainfall is not a good estimator for tree diversity in the Amazonia area and Guianashield. Plots in the Guiana Shield area (and eastern Amazonia) usually have lower diversity than those in central or western Amazonia. It is argued that this is not because of low rainfall or low nutrient status of the soil but because of the small area of the relatively isolated rain forest area in eastern Amazonia and the Guiana Shield. The low diversity on nutrient-poorwhite sand soils in the Amazon basin is not necessarily due to their lownutrient status but is, at least partly, caused by their small extent and fragmented nature.