Clay dispersal is one of the most promising strategies for controlling harmful algal blooms. It is based on the mutual aggregation of algal cells with mineral particles, leading to aggregate settling. This research demonstrated the effectiveness of domestic clays against bloom-forming species from the United States (> 90% removal efficiency, RE), such as Karen ta brevis and Heterosigma akashiwo, at clay loadings < 0.25 g 1-1. Algal viability and recovery depended on loading, resuspension frequency, and contact duration between clay and cells before the first resuspension event. Phosphatic clay showed varying RE against seventeen species from five algal classes, and removal trends varied with increasing cell concentration. RE was not correlated with algal size, swimming rate, and the type of cell covering. RE was correlated with the predicted collision frequency coefficient. Selective removal of K. brevis by phosphatic clay was observed in mixed laboratory cultures, and in mesocosms containing a natural field assemblage. Marine microalgae and clay minerals in natural seawater displayed a narrow range of negative electrophoretic mobilities (EPM). EPM values did not correlate with observed removal patterns. Kinetic studies of the clay-cell system revealed varying rates of aggregation and settling among bentonite, kaolinite and phosphatic clay with K. brevis.