The region of Guaratiba-Sepetiba, about 50 km WSW of Rio de Janeiro, represents a model of two interdependent mixohaline environments, the Mangrove of Guaratiba and the Bay of Sepetiba, both in equilibrum between the continental and the open marine influences. The distinction between these mixohaline environments is based on abiotic parameters and on the biocoenoses of the foraminifera. The high degree of restriction of the mangrove environment produces an endemic, diversified and specialized association of foraminifera consisting essentially of Textulariids. The Bay of Sepetiba is open toward the marine environment and contains low diversified and restricted foraminiferal associations consisting essentially of Rotaliids. The diversity cœfficient is about 3 for the foraminiferal associations of the Bay and goes from 1,5 to 10 in the Mangrove. The high diversity coefficients (χ = 6 to 10) which occur in the mangrove environment do not reflect the degree of restriction but the level of adaptation of the foraminifera to the mangrove environment. In this respect the high diversity cœfficients are comparable with those characterizing normal marine shelf faunas.