We investigated the possibility of using NaOCl produced by electrolysis of natural seawater to control the outbreak and maintenance of red tides. The densities of the red tide dinoflagellates Cochlodinium polykrikoides, Gymnodinium sanguineum, Lingulodinium polyedrum, Gyrodinium impudicum, Scrippsiella trochoidea rapidly reduced > the concentrations of 0.5 ppm in the laboratory experiments. However, the density of a naked ciliate Strombidinopsis sp. rapidly reduced > the concentration of 1 ppm and that of a calanoid copepod Acartia sp. was not affected < the concentration of 10 ppm. Half the concentration of NaOCl was converted to NaCl within 2 hours under bright sunlight. Therefore, NaOCl produced by electrolysis reduce the population sizes of red tide dinoflagellates with minimizing the secondary effects on the populations of ciliates and copepods at the concentrations of 0.5 ppm.