The infuence of sodium fuoride (NaF) on cal-cium metabolism was examined in nibbler fsh (marine tel- eosts). Two days after the administration of NaF (5 μg/g of body weight) (around 10−4 M in fsh), we showed that plasma calcium levels signifcantly decreased in NaF- treated nibbler fsh. In addition, we detected fuoride in the treated scales by use of a scanning electron microscope with an energy-dispersive X-ray microanalysis, indicating that NaF directly affects their scales. Therefore, the infu-ence of NaF on osteoblasts and osteoclasts in the scales was examined. In the scales of NaF-injected nibbler fsh, tartrate-resistant acid phosphatase (TRAP) (osteoclastic marker enzyme) decreased, although alkaline phosphataseosteoblastic marker enzyme) was activated. To confrm the effect of NaF on osteoclasts, furthermore, the mRNA expressions of osteoclastic markers (matrix metallopro-teinase-9 and TRAP) were decreased signifcantly 2 days after incubation. In barred knifejaws, plasma calcium levels decreased as they did in nibbler fsh. Therefore, NaF func-tions in both osteoblasts and osteoclasts and then infuences calcium metabolism in marine fsh. In the marine environ-ment, high levels of fuoride (1.2–1.5 mg F−/l) (around 10−5–10−4 M) are present in seawater. It is probable that teleosts living in seawater effciently use fuoride to regu-late their blood calcium levels.