Changes in the biomass of Amazon region forests represent an important component of the global carbon cycle but the biomass of these forests remains poorly quantified. We examined forest survey data for trees with a diameter at breast height (DBH) greater than 35 cm from 4 plots with a total area of 392 ha in the Tapajos National Forest near Santarem, Para, Brazil (S 3.04, W 54.95). The average frequency of trees greater than 35 cm DBH was approximately 55 ha^-1. Based on tree diameter data, allometric relations, and published relations for biomass in other compartments besides trees of DBH > 35 cm, we estimated a total biomass density of 372 Mg ha^-1. Trees with diameters greater than 35 cm DBH accounted for about half of the total biomass. This estimate includes all live and dead plant material above and below ground with the exception of soil organic matter. We propagated errors in sampling and those associated with allometric relations and other ratios used to estimate biomass of roots, lianas and epiphytes, and necromass. The major sources of uncertainty in our estimate were found in the allometric relations for trees with DBH greater than 35 cm, in the estimates of numbers of trees with DBH less than 35 cm, and in root biomass. In the worst case, we estimate an uncertainty in this value of about 40%. Simulated sampling based on our full survey, suggests that we could have estimated mean biomass per hectare for trees (DBH greater than 35 cm) to within 20% with 95% confidence by sampling 21 randomly selected 0.25 ha plots in our study area.