Abstract— Migrations of fish in the outlet channel of Lago do Rei, a floodplain lake on Careiro Island in the Amazon River, were examined using three capture methods. Of the 85 species found in the Parana do Rei (channel), 20 migrated between the lake and river. Migrations from the lake to the river occurred in two pulses. In August-September, as the river water level began to recede, some fish migrated to the river with the current. As the river water level rose in December-January, additional fish migrated towards the main river against the current. Fish returned to the lake by March or April after they had spawned. The migratory patterns of six species, family Curimatidae and Prochilodontidae: Potamorhina latior, Psectrogaster amazonica, P. rutiloides, Curimata kneri, Prochilodus nigricans and Semaprochilodus taeniurus are described in detail. Lateral migrations of fish in this ecosystem appear to be closely linked to breeding and to physical habitat changes related to river water level.