The Bigelow Lab in-flow particle imaging and analyzing system was designed with a 3 micrometer resolution and a maximum particle size of 1 mm, making it useful for studying marine particles well beyond the useful range of conventional flow cytometers. This system allows the user to continuously monitor phytoplankton and particulate matter in underway water or sampled water over long periods of time. When particle fluorescence is detected, the fluorescence and image data is acquired, stored, and presented to the user in scattergram and image form.While this system emulates conventional flow cytometers, it has a flow rate twenty times greater, making it useful for studies of sparse larger phytoplankton. It also provides images of each cell event, making it useful for particle classification. Because of these features, this system allows rapid analysis of particles which affect optical variability and photophysiological processes. The entire system is inexpensive, compact and PC-based and uses a novel binary optical element to increase the imaged volume. We present data from samples from cruises and from locally sampled water.