An inventarisation of the plankton living in an old branch cut off from the river Waal has been made on August 16, 19 and 23, 1955. In the deep part lake plankton (Limnoplankton) was found, and in the shallow parts pond plankton (Heleoplankton) occurs. Some rare plankters typical for oligotrophic waters were found in a certain part of the river branch. A chemical analysis of this part of the water shows no real difference with other parts. The part of the old branch connected with the river Waal has a plankton community of its own. This is caused by the polluted river water. The horizontal distribution of plankton on August 16, 1955, was investigated. The vertical distribution of plankton investigated on August 28, 1941, is also given.