We present results on the effect of modified clay on cyst formation of Scrippsiella trochoidea in harmful algal bloom(HAB).Modified clay(in concentration of 0,0.1,0.5,and 1.0 g/L) were added to cultures,and observations were made on cysts of 5.trochoidea under controlled laboratory conditions.Results indicate that the removal rate of algal cells reached 97.7%at the clay concentration of 1.0 g/L.The cyst formation rate increased from 4.6%to 24.6%when the concentration of clay was increased from 0to 1.0 g/L.Two cyst metamorphs were observed:spinal calcareous cysts and smooth noncalcareous ones.The proportion of the spinal cysts decreased from 76.9%to 24.1%when clay concentration increased from0 to 1.0 g/L.In addition,modified clay affected cyst germination.The germination rate decreased with the increases in the clay concentrations.Non-calcareous cysts had a lower germination rate with a longer germination time.We conclude that modified clay could depress algal cell multiplication and promote formation of temporal cysts of 5.trochoidea,which may help in controlling HAB outbreaks. We present results on the effect of modified clay on cyst formation of Scrippsiella trochoidea in harmful algal bloom (HAB) .Modified clay (in concentration of 0, 0.1, 0.5, and 1.0 g / L) were added to cultures, and observations were made on cysts of 5. trochoidea under controlled laboratory conditions. Results indicate that the removal rate of algal cells reached 97.7% at the clay concentration of 1.0 g / L. The cyst formation rate increased from 4.6% to 24.6% when the concentration of clay was increased from 0 to 1.0 g / L. Two cyst metamorphs were observed: spinal calcareous cysts and smooth noncalcareous ones. the proportion of the spinal cysts decreased from 76.9% to 24.1% when clay concentration increased from 0 to 1.0 g / L. In addition, modified clay affected cyst germination.The germination rate decreased with the increases in the clay concentrations. Non-calcareous cysts had a lower germination rate with a longer germination time. We conclude that modified clay could depress algal cell multiplication and pr omote formation of temporal cysts of 5.trochoidea, which may help in controlling HAB outbreaks.