This study examined the flocculation and sedimentation of four HAB species (Gymnodinium breve, Heterosigma akashiwo, Pfisteria piscicida, and Aureocuccus anophagefferens) using a variety of clays. More than 30 clays were tested and these could be grouped into five categories ranging from high to low removal efficiency. Clays that removed one species well were not always effective on other species. 80%-90% cell removal could be achieved with loadings as low as 30 ppm in some cases (e.g. Gymnodinium and Heterosigma), but with other species and clays, 250 ppm or higher loadings were needed. Surfactants such as polyaluminum chloride (PAC) reduced the loading needed for cell removal by nearly an order of magnitude. Moreover, cell removal efficiency increased significantly when the clay-cell suspension was agitated (in the case of Aureococcus), but removal was less efficient with increasing salinity (in the case of Pfiesteria) and higher cell concentration. Experiments in large tanks and settling columns demonstrated that higher amounts of clay than predicted by simple scaling factors from test tube experiments will be necessary to maintain comparable removal rates. Clay additions using phosphatic clay resulted in 80% mortality of precipitated Gymnodinium cells at 500 ppm. No cell growth was observed six days after treatment. While clays can both release and remove important algal nutrients (NO3/NO2, NH4, PO4 and SiO4) in the water column, laboratory studies have shown that the release can be moderated considerably by the addition of small amounts of PAC. Lastly, studies on the impact of clays on the benthic and planktonic communities are in progress and will be discussed.