A recording fluorometer provides a rapid, automatic way to estimate sinking rates of freshwater phytoplankton, the determinations taking from 15 min for populations sinking at 1.0 m day−1 to 2 h for populations sinking near the limit of detection, 0.1 m day−1. The shape of the fluorescence output provides information on the distribution of sinking rates about the mean. Measured sinking rates of Asterionella formosa Hass. agree well with published data. Sinking rates of fluorescent micronic beads at various temperatures agree with Stokes’ law. The technique is applicable to marine phytoplankton, reducing the time needed for a determination two to five times over existing methods and so increasing the sensitivity that measurements may be directly possible for natural water samples.