This paper presents some results of a five year monitoring programme on Zooplankton in the River Rhine at the Dutch sampling stations Lobith (German/Dutch border) and Maassluis (at the discharge of the river into the North Sea). During the entire period the Zooplankton composition showed an apparent seasonal pattern at both stations. This pattern was characterized by low densities during the winter and higher densities during the summer with an obvious spring peak. Zooplankton was dominated by rotifers with Brachionus angularis, B. calyciflorus, Keratella quadrata and K. cochlearis as dominant species. The possible role of environmental factors (flow rate and chlorophyll-a content) as controlling factors for Zooplankton dynamics is indicated.