Abstract: Ocean acidification will decrease the saturation state of calcium carbonate and negatively affect a range of calcareous taxa.Some fishes, such as marine medaka Oryzias melastigma, produce otolith structures that are mainly composed of aragonitic CaCO₃, which may be sensitive to elevated levels of pCO₂.It has been particularly concerned that ocean acidification impacts on the development of otolith in fish embryos and larvae.Nevertheless, the effects of ocean acidification on calcification of otolith in adult fish have few reports.In this study, the potential effects of an increasing CO₂ on the morphology and composition of the otoliths were investigated in mature medaka.Four-month old individuals were raised during 45 days under four CO₂ concentration conditions (450 ppm, 980 ppm, 2900 ppm and 4850 ppm) that are predicted to occur over the next 100 to 300 years.Medaka exhibited somatic growth resilience to all treatments.There are three pairs of otolith in medaka's inner ears.They are sagittae, asteriscus and lapillus, respectively.The shape of otolith (including surface area and roundness) was not affected by ocean acidification.However, the weight of sagittae decreased with the increase of CO₂ concentration.In 2900 ppm and 4850 ppm treatment groups, the weight of sagittae was significantly lower than that in the control group.There were obvious asymmetry in otolith surface area and otolith roundness between left and right sagittae of medaka reared in 2900 ppm treatment groups.In higher CO₂ treatment groups, the relative content of C in sagittae was significantly higher than that in control group.The changes of otoliths due to elevated CO₂ concentration could have consequence on auditory sense and relating behaviors, such as predation, avoidance of predators.