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,Oryzias latipes. Both sagittae were partly vateritized in 70% of individuals subjected to the Sr2+treatment (n= 10), whereas fish reared in normal tap water showed no sagittal vateritization (n= 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr2+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 Sr2+and higher Mg2+concentrations than the aragonitic phase. It is unlikely that increased environmental Sr2+is responsible for the sagittal vateritization in farmed fish. However, our findings likely help to establish anin vivoassay usingO. latipesto understand the physiological process underlying the sagittal vateritization in farmed fish.