Beam attenuation and single-cell light scattering have often been observed to increase during the day and decrease at night in surface waters, presumably due to phytoplankton cell growth and division. At the Coastal Mixing and Optics (CMO) site on the continental shelf south of Cape Cod, Massachusetts, changes in the size of phytoplankton cells over the diel cycle are apparent, despite advection and mixing of the water column. In August-September 1996 and in April- May 1997, depth profiles were sampled 1 to 3 times per day for hydrography, transmissometer measurements of beam attenuation, and flow cytometric measurements of concentration and individual cell forward light scattering for phytoplankton cells (the picoplankton Synechococcus and eukaryotic phytoplankton in 5 ml samples). In addition, several more frequent samplings were performed over the daylight hours on each cruise and during spring 1997 an intensive 24-h sampling was carried out. Forward light scattering was converted to cell volume and to carbon per cell using laboratory and published calibrations. During the late summer, the mean volume or carbon content of near-surface cells was observed to increase by about two-fold over the course of a single day. This variation was nearly as large as that observed during the entire 3-week sampling period at a given time of day. During the spring, mean cell volume also varied by about a factor of two over the one intensively sampled diel cycle. However, over the spring sampling period variability was much greater, largely due to a phytoplankton bloom. Despite changes in cell concentrations due to factors such as grazing, mixing and water mass movements, similar patterns could be seen in the bulk optical property of beam attenuation.