During the year 1980, monthly observations were made on the distribution and dynamics of fish populations in two mangrove lagoons (Belle-Plaine and Manche-a-Eau). A total of 39 species was found in the two mangrove lagoons but the maximum number observed did not exceed 19 species for each sampling. The species were separated into four main groups: (1) the sedentary species which spend their whole life in the mangrove lagoons; (2) the species only represented by juvenile individuals (the greatest group); (3) the species occurring occasionally in the mangrove lagoon; (4) the species only observed during the reproduction period. Three demographic models were tested: the models of Preston, Motomura and MacArthur. Preston better fitted the population of Belle-Plaine (better value of the distance of Matusita). Motomura better suited the population of Manche-a-Eau, which can be explained by the dominance of a sedentary species, which adapts better to difficult ecological conditions.