The large range in concentrations and cell-sizes of algal cells and colonies and the large variety of cell types are the main reasons for developing a dedicated cytometer for the analysis of phytoplankton. A European Community funded consortium has developed the EurOPA cytometer, which is easily transported and can be operated at sea. With the EurOPA, both small single cells and large colonies of cyanobacteria can be analyzed in one run. This provides correlated information on optical characteristics, pigments contents and taxonomy. The resulting distribution of (chlorophyll) biomass over taxonomic groups can be inter-calibrated with standard spectrometric analysis techniques. The EurOPA can be used successfully for analysis of field samples and phytoplankton cultures. It is well suited for phytoplankton monitoring and grazing studies.