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Rhizophora mangle L. (Rhizophoraceae) grows on aerial roots, which emerge above the water level, giving stands of this tree the characteristic "mangrove" appearance. To produce in situ and ex situ information for genetic conservation programs for this species, we investigated the genetic diversity and mating system of one R. mangle population. We sampled 30 adult trees and a total of 349 seeds in Northern Brazil. We genotyped all adults and seeds with four microsatellite loci. The average fixation index was -0.222 for seed trees and 0.030 for seeds. The multilocus outcrossing rate (=0.921) was significantly lower than unity (1.0). There was no substantial evidence of null alleles and genotypic disequilibrium among the loci. The combined power to exclude the first parent probability was 0.921. The average coancestry coefficient (=0.180) was similar to that expected for half-sib progenies (=0.125). Thus, the number of seed trees necessary for seed collection to obtain progeny arrays with an effective size of 150 was estimated to be 62. In conclusion, this study produced important information for the management and conservation of R. mangle and will contribute to conservation and management programs for this species.