The hematocrit value of fish blood is known to easily vary with physiological conditions of the fish. The relationships between gas contents and hematocrit value were examined in the yellowtail Seriola quinqueradiata to know respiratory abilities of the fish at various hematocrit conditions. The oxygen capacity, oxygen content and oxygen saturation in the arterial blood, and the oxygen content and oxygen saturation in the mixed venous blood increased with hematocrit elevation. The regression equations of these relationships were obtained. The oxygen utilization at the tissues markedly elevated in fish of low hematocrit values. This phenomenon is considered to be an adjustment to maintain the oxygen supply to the tissues at fairly good levels under lowered oxygen transport of the arterial blood caused by low hematocrit values. The carbon dioxide contents both in the arterial blood and in the mixed venous blood were fairly constant over a wide range of hematocrit value.