Amazonian forests play a key role in the global carbon cycle but there is much uncertainty about the quantity and distribution of carbon stored in these forests. We quantified total aboveground dry biomass (TAGB) in undisturbed central Amazonian rainforests based on detailed estimates of all live and dead plant material within 20 1 ha plots spanning an extensive (ca. 1000 km2) study area. TAGB values in our study area were very high averaging 397.7±30.0 Mgha−1. The most important component of aboveground biomass was large (≥10 cm diameter-at-breast-height (DBH)) trees which comprised 81.9% of TAGB followed by downed wood debris (7.0%) small trees saplings and seedlings (<10 cm DBH; 5.3%) lianas (2.1%) litter (1.9%) snags (1.5%) and stemless palms (0.3%). Among large trees aboveground biomass was greatest in intermediate-sized (20–50 cm DBH) stems (46.7% of TAGB) with very large (≥60 cm DBH) trees also containing substantial biomass (13.4% of TAGB). There were no significant correlations between large tree biomass and that of any other live or dead biomass component. An analysis based on the variability of our samples suggested that just 3–4 randomly positioned 1 ha plots would be sufficient to provide a reasonable estimate of mean TAGB in a landscape such as ours (with 95% confidence intervals being <10% of the mean). This suggests that efforts to quantify Amazon forest biomass should be extensive rather than intensive; researchers should sample many geographically separate areas with a few plots each rather than sampling a small number of areas more intensively.