Recent developments in optical sensor technology have opened new opportunities for measuring particle size distribution in-situ. Currently, there are only few commercially available in-situ particle size analyzers; one of them is the Laser In-Situ Scattering and Transmissometry sensor (LISST, Sequoia Scientific Inc.). The LISST measures the near-forward light scattered by particles and the signal is inverted to obtain particle size distribution. The principle of inversion to size is based on light diffraction, which is relatively insensitive to particle composition. A major assumption in the inversion, however, is that particles are homogeneous spheres. While it is widely acknowledged that aquatic particles are generally non-spherical, there are very few studies that examined effects of deviation from the spherical assumption on size measurements. Of particular interest is how well the LISST performs in sizing phytoplankton cells, which frequently dominate the particulate pool but exhibit diverse cell morphologies. We conducted laboratory measurements in which we compared size and volume concentration distributions of different phytoplankton cultures obtained by the LISST-100 to those derived from microscopy. For spheroids or any near-spherical shaped cells, the LISST-100 provides particulate volume and area size distribution that is similar to that derived from microscopy. For more complex shapes, such as cells with spines or chains (e.g., Ceratium, Skeletonema), LISST-100 measurements reflect the cross-sectional areas of the different orientations of the cells presented to the light beam. We will also present data that demonstrate the potential utility of the LISST in laboratory studies of life cycle of diatoms where monitoring of changes in cell size is essential. The utility of the LISST in providing added information for in-situ analysis of phytoplankton blooms, and in particular with harmful species, will be discussed.