An image analysis algorithm for extracting particle size distribution information from in situ images of suspended, high-aspect-ratio crystals is described. The accuracy of the algorithm is assessed using comparisons with results obtained by manually sizing the imaged crystals. The algorithm can accurately extract crystal size information from in situ images of suspended, high-aspect-ratio crystals for moderate solids concentrations. The algorithm's performance declines somewhat for high solids concentrations and high levels of particle attrition. The speed with which the algorithm analyzes the images is suitable for real-time monitoring and control of particle size distribution mean and variance. (c) 2006 Elsevier Ltd. All rights reserved.