Sagittal otoliths, typically composed of aragonite, are frequently laid down rather as vaterite during growth in hatchery-reared fish populations. Sagittal vateritization is believed to impair individual hearing/balancing abilities, but the causal mechanism remains unclear. Here we experimentally demonstrated that rearing in Sr-rich water induces sagittal vateritization in the HdrR-II1 inbred strain of the Japanese rice fish, . Both sagittae were partly vateritized in 70% of individuals subjected to the Sr treatment ( = 10), whereas fish reared in normal tap water showed no sagittal vateritization ( = 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr concentration in solution increases. A vateritic layer develops surrounding the original aragonitic sagitta in vateritized otoliths, some of which take on a comma-like shape. Electron probe microanalysis demonstrates that the vateritized phase is characterized by lower Sr and higher Mg concentrations than the aragonitic phase. It is unlikely that increased environmental Sr is responsible for the sagittal vateritization in farmed fish. However, our findings likely help to establish an assay using to understand the physiological process underlying the sagittal vateritization in farmed fish.