Calegario, G.; Salomão, M.S.M.B.; de Rezende, C.E., and Bernini, E., 2015. Mangrove forest structure in the São João River estuary, Rio de Janeiro, Brazil.Natural and anthropic factors contribute toward the formation of environmental gradients that act as selective forces that in turn determine the distribution of the mangrove species along these gradients. The aim of this study is to characterize the structure of two mangrove forests under different conditions of salinity and compare the structural parameters between the border and inside forests in the São João River estuary. One site 11 km from the ocean (upper estuary, site 1) and another 3 km from the ocean (lower estuary, site 2) were selected along the estuary. Within each site measurements of height and diameter breast height (DBH ) of individuals ≥1 m in height, and the pore-water salinity, granulometry, and organic matter of the sediment were analyzed. The average tree height varied from 3.1 to 6.1 m, the average DBH from 3.7 to 7.0 cm, the density from 5240 to 14,720 trunks ha−1, and the basal area from 8.0 to 35.5 m2 ha−1. A pattern of zonation was observed along the estuary, with Laguncularia racemosa (L.) Gaertn. f. dominant in the upper estuary and Rhizophora mangle L. dominant in the lower estuary. We observed no zonation of species between border and inside forest, but L. racemosa exhibited higher density in the upper estuary, and R. mangle in the lower estuary, thus defining a pattern of spatial zonation along the estuary.