ABSTRACT: Present automated systems for counting and measuring marine plankton include flow cytometers and in situ plankton video recorders. Neither of these approaches are optimal for the microplankton cells which range in size from 20 to 200 µmand can be fewer than 10⁴ l^-1. We describe here an instrument designed for rapid counting, imaging and measuring of individual cells and particles in the microplankton size range from cultures and natural populations. It uses aunique optical element to extend the depth of focus of the imaging lens, allowing a sample stream flow rate of 1 ml min^-1. The instrument stores a digital image of each particle along with real time fluorescence and size measurements. Aninteractive cytogram links a dot-plot of the size and fluorescence data to the stored cell images, allowing rapid characterization of populations. We have tested the system on live phytoplankton cultures and bead standards, proving the system counting andsizing accuracy and precision. The system provides images and size distributions for cultures or natural marine samples. It has been used successfully at sea to continuously monitor particles while underway. It may prove useful in studies of planktoncommunity structure, ocean optics and monitoring for harmful algal species.