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Centre ORSTOM BP 165, 97323 Cayenne Cedex, French Guiana Abstract The Sinnamary is a small tropical river (250 km long, watershed of 6500 km 2 ) where a dam is to be impounded in 1994 .The extent of naturally periodically flooded areas along the Sinnamary river is estimated by using remote sensing (Landsat TM satellite data) and botanical groupings are described on in situ observations, ground truth and collections . The fish assemblages of these areas are described and their density estimated by rotenone sampling . The role of this ecotone to fish populations is discussed .The extent of the inundated areas appears to be greater than previously estimated by classical mapping .As expected, the fish fauna is mostly composed of fish juveniles and adults of dwarf (< 3 cm) and small (< 15 cm) species . Large predators where not found . Due to these characteristics, fish are abundant but their biomass is low in naturally inundated areas . However, juveniles of some species found only in these zones . Therefore their adult populations will probably be very sensitive to any modification of these land/water ecotones .