We report on lateral movements of fish between an Amazonian headwater river (Manu River in Manu National Park, Perú) and a 24-ha oxbow lake (Cocha Cashu) in the adjacent floodplain. During wet season flood pulses, or 'crecientes,' fish can enter and exit the lake through a connecting channel that is normally dry. To investigate fish movements, we operated a bi-directional funnel trap in the connecting channel during high water interludes. We captured 4090 fish of 60 species during the 2005 and 2006 rainy seasons and sub-sampled the adults for gonadal state and stomach contents. We found that most exiting fish were gravid and that most entering fish had recently spawned, suggesting that fish were leaving the lake to spawn in the river or elsewhere. Entering fish had full stomachs more often than exiting fish and entering and exiting individuals of most species were of similar size.