We examined effect of aquatic hyperoxia on respiration of yellowtails exposed to Chattonella marina and measured relevant physiological parameters of the fish during the exposure. Water O₂ saturation level was elevated from 100% to 300% when arterial Po₂ (Pao₂) was found to have declined below 30mmHg during the exposure. In the first experiment (experiment 1, N=7), Chattonella exposure lasted 210 min, followed by a recovery period of 210 min in normoxic water free of Chattonella cells. In the second experiment (experiment 2, N=7), Chattonella exposure lasted 420min in hyperoxia. Survival rates of yellowtail of the two experiments were 100% and ca. 60% (four fish survived, three fish died) in experiment 1 and 2, respectively. In both experiments, hyperoxia failed to elicit expected large increases in Pao₂, which suggests that resistance to diffusion of oxygen through the respiratory epithelium was increased by the presence of Chattonella cells. Ventilatory pressure (VP) was consistently enhanced when Pao₂ was initially depressed in normoxia, but such stimulation of VP did not always occur during hyperoxia even though Pao₂ was often depressed under this condition. The gill showed variable degrees of lesions.