Biomass content and turnover rate were estimated for a lowland wet rain forest in French Guiana. A regression model relating the biomass of a tree to its dbh (diameter at breast height) was deduced from previously published data. A power-law allometric relationship of the form AGTB = aDb was used to estimate the tree biomass, AGTB (Mg ha−1), from its dbh D (cm). Using direct measurements of tree biomass in the literature, the best-fit allometric exponent b = 2.42 (SD = 0.02) was found. The logarithm of the coefficient a was normally distributed with an average of −2.00 (SD = 0.27). This method was applied to two permanent research stations of the lowland tropical rain forest of French Guiana: the Nouragues and Piste de Saint-Elie. At the Nouragues, the biomass was estimated from trees 10 cm in diameter on two plots covering a total surface area of 22 ha and yielded an average biomass of 309 Mg ha−1 (± 32 Mg ha−1, 95% confidence interval). Spatial variability was also addressed at the Nouragues by estimating the biomass of trees ≥ 30 cm dbh over a total surface area of 82 ha. For the wet tropical forest vegetation type, an average of 284 Mg ha−1 was obtained (spatial variability ±55 Mg ha−1). Biomass turnover was evaluated at Piste de Saint-Elie from two transects (0.78 and 1 ha) on which all trees ≥5 cm in diameter were recorded and mapped twice in 10 y. Transect 1 showed a slight increase in biomass, from 245 to 260 Mg ha−1 (338 to 345 Mg ha−1 for transect 2), corresponding to a net increase of 1.9 Mg ha−1 y−1 (0.7 Mg ha−1 y−1), and the biomass ingrowth was 3.2 Mg ha−1 y−1 (2.8 Mg ha−1 y−1). These figures are discussed in the light of the natural recruitment dynamics of tropical forests.