We discuss an in situ marine imaging system based on high-speed digital line scan cameras for collection of a continuous picture of microscopic marine particles ranging in size from 200 µm to several centimeters. The system is built to operate on a 53-cm-diameter autonomous underwater vehicle or a tethered platform. The digital imaging system provides a continuous record of all particles passing through a symmetric 96 x 96 mm sampling tube and provides views from two orthogonal directions. Data are compressed using a lossless encoding technique and stored onto a disk drive. Over 50 h of continuous imaging is possible using the system. Data are suitable for studies requiring sizing, identification, quantification, and spatial recording of semi-transparent and opaque particles. This paper summarizes the mechanical, optical, and data processing design of this instrument and discusses recent improvements.We also present images and quantitative results from recent deployments in the Gulf of Mexico.