A new video-microscope has been developed for in situ investigation on marine particles. A field of view is produced by a laser beam at 473 nm. Individual cells or particles that entered this field of view appear as individual diffraction-limited spots of light, which are resolved from the dark background. A mobile high-pass optical filter (610 nm) can be controlled and displaced in front of the CCD camera, allowing simultaneously imaging and discrimination between fluorescing and non-fluorescing particles. We have the possibility to add a band pass filter for phycoerytrin detection. The system allows white light visualisation of particles ranging from 10 µm to several millimetres, depending on the zoom magnification. With the 610 nm high pass filter, the light detected by the CCD is mainly due to chlorophyll fluorescence (phytoplankton). Image processing allows to extract information on the particle characteristics such as size, shape, number or movement trajectories. In coastal ecosystem studies, this new device is well adapted to the description of phytoplankton populations variability. Integrating this new technique on conventional profiler will make possible a new sampling strategy for toxic dinoflagellates study.