Teleost fish live under a constant force derived from gravity, with hard tissues playing important roles to help maintain body balance. However, the mechanism of hard tissue formation induced by gravity remains unclear. To examine the effects of gravity in aquatic animals, we performed experiments with medaka fish reared in a hypergravity environment, in which the force of gravity exceeded that present on the surface of the Earth, and analyzed hard tissue formation. Medaka fish were reared for 6 months under a normal gravitational force (1G) or that 5.29 times greater than normal (5.29G) using a centrifuge designated for small fish rearing. Micro-CT analysis results showed that hypergravity induced a vertebral curvature towards the dorsal side and asymmetric formation of otoliths in which the crosssectional area was increased. Our findings indicate that the process of adaptation to a hypergravity environment results in spinal and otolith deformation in medaka fish. ©2019 Jpn.