The design criteria for a flow cytometer (FCM) for the analysis of field samples of phytoplankton are described. The criteria are based on the occurrence of a wide variety of particle sizes in field samples, normally at low concentrations. The instrument should be able to analyse cells and colonies from 0.5 to 500 microns diameter and of over 2,000 microns length. A minimum flow rate of 4 microliters.s-1 was calculated from natural plankton concentrations. Commercially available FCMs are not suited to measure this range of sizes at this rate. Further limitations of standard FCMs are uneven illumination or incomplete processing of long signals. In addition, long filamentous colonies can break into small fragments caused by too high acceleration in the standard flow cuvette. Recognition of these limitations is of importance for the flow cytometry of phytoplankton. The new design was developed to avoid these limitations. A dynamic range 5 to 6 decades could be accomplished by a combination of logarithmic amplifiers, a slit-shaped focal spot, and a pulse integration system that can process long pulses. Multilaser capability to identify different phytoplankton species, a low fluid shear cuvette, and a trigger gate-extension for inhomogeneously fluorescent algal filaments were included in the design.