The sublethal and lethal effects of chlorine produced oxidants (CPO) on juveniles of the estuarine teleost, Leiostomus xanthurus, were investigated in flowing water tests conducted at 30 plus or minus 1C and 26 to 31% salinity. Short-term LT50 tests were conducted at two nominal concentrations of NaOCl 1.0 and 1.4 mg/l (respective measured CPO concentrations 0.09 and 0.12 mg/l) which were sublethal in 2,880 minute exposures; and three nominal concentrations, 1.6, 1.8, and 3.2 mg/l NaOCl (respective measured CPO concentrations 0.13 and 0.37 mg/l) which were acutely toxic. Opercular ventilation rates in exposed spot were much higher than in control fish, but returned to rates only slightly above those of controls during the latter portion of the 2,880 minute exposure of the two sublethal CPO concentrations. Opercular rates at the three acutely toxic CPO concentrations remained much higher than control rates until the exposed fish died. Oxygen uptake by spot was depressed at all the measured concentrations of CPO tested. Histopathological examinations showed that gill respiratory ephithelial tissues sloghed away from the underlying pillar cells. Complete denudation of circulatory tissues and hemangiectic secondary lamellae were observed in gill tissues from fish exposed to the highest CPO concentrations of 0.37 mg/l.