Sodium hypochlorite (NaOCl) has been demonstrated to be effective against algae that cause red tides. To secure the environmental safety of this chemical for practical use, toxic effects of NaOCl were evaluated in two important aquaculture fishes, flounder and rockfish. At concentrations above 2.5 and 5 ppm for 1 hr, mortality occurred in flounder and rockfish, respectively. Flounder, however, did not die at 2 ppm up to an 8 hr exposure, nor did lipid peroxide level change at the same concentration in 1 hr. Plasma glucose level significantly increased in 1 hr with more than 2 ppm exposure. The results indicate that NaOCl may not induce persistent severe toxicity in flounder and rockfish at the algicidal concentration, 0.5 ppm.