Acid treated clay(ATC)was developed as an algicide. The ATC requirement for a 90% removal of marine plankton, Oisthodiscus sp., Chattanella sp., Dunaliella sp., and Skeletonema costatum, were observed to be a maximum 220mg/l at pH 7-8, and decrease in either lower or higher of the pH range. Natural sca waters blooming red tide organisms (34 and 218μg chlorophyl a/l)is Tokyo Bay required 130 and 250mg/l of ATC. The minimum and maximum release of aluminium from the clay into sea water were shown at pH 6.7 and 8.5 respectively. Where approximately 100% of clay particle was coagulated down between pH 6.8 and 8.3 re-gardless of ATC concentration. It was suggested that cell removal mechanism of ATC may be mainly due to disruption of cells followed by coagulation with positive charged aluminium hydroxide under about pH 7. Flocculation with hydroxide of Al and Fe may also play im-portant role in removal of cells.