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A better understanding of the reasons for variation in tropical rain forest (TRF) structure is important for quantifying global above-ground biomass (AGBM). We used three data sets to estimate stem number, basal area, and AGBM over a 600-ha old-growth TRF landscape (La Selva, N.E. Costa Rica). We analyzed the effects of soil type, slope angle, topographic position, and different sample designs and measurement techniques on these estimates. All three data sets were for woody stems ≥10cm in diameter. Estimated AGBM was determined from stand-level measurements using Brown’s (Brown, 1997) allometric equation for Tropical Wet Forest trees. One data set was from three subjectively-sited 4-ha plots (the ‘OTS plots’), another was based on 1170 0.01ha plots spaced on a regular grid (the ‘Vegetation map plots’), and the third was from 18 0.5ha plots (the ‘Carbono plots’) sited to provide unbiased samples of …