Quantitative plankton observations have been carried out in the River Rhine from Lake Constance to the North Sea as part of an ecological monitoring programme, initiated to provide a baseline for evaluating measures taken under the “Rhine Action Programme”. The phyto- and Zooplankton monitoring, carried out in 1990 is the first concerted programme covering the 1000 km long reach of the river below Lake Constance. Seasonal observations at 20 fixed stations along the river were carried out by different institutes; this required calibration of methods. In the upper Rhine the sparse population of phyto-plankton (< 5 µg chlorophyll-a l-1) was dominated by various groups of algae, i.e. Cryptop-hyceae, Bacillariophyceae, Chlorophyceae and Cyanophyceae. Downstream, the phyto-plankton density increased gradually, while Bacillariophyceae tended to dominate. The high densities found in the lower R. Rhine represented only a few species of centric Bacillariophyceae and Chlorophyceae. The dense blooms of phytoplankton in the lower R. Rhine are triggered by the input of phytoplankton from the upper reaches and tributaries, and are stimulated by large inputs of nutrients. These blooms are accompanied by blooms of Zooplankton, especially rotifers, Dreissena polymorpha larvae and, in the lower Rhine, nauplii larvae. The species composition and biomass of plankton in the R. Rhine vary strongly during the year but also between years, as indicated for the period 1980-1990. However, the information at the species level needs further elaboration to allow interpretation of long-term trends. Notably, the input by tributaries should be included. To assess these long-term trends we recommend supplementing the large-scale biological inventory, as planned for the years 1995 and 2000, with plankton observations in the years between and investigating the relationship between the benthic and pelagic communities of the river.