The Japanese dace (Tribolodon hakonensis) is the only teleost species that inhabits Lake Osorezan, a remarkable acid lake in Japan with the water pH of 3.4–3.8. In the present study, physiological changes following transfer of the dace acclimated to neutral stream water (pH 6.8–7.2) to acid lake water (pH 3.6–3.7) were examined with special reference to changes in gill chloride cell morphology. The dace survived direct transfer to acid lake water for 3 days. Plasma [Na ] and [Cl⁻] showed transient decreases at 1 hr after transfer; however, the decrease in [Na ] was greater than that in [Cl⁻]. The recovery of [Cl⁻] was more evident than that of [Na ]. The transient decreases of plasma [Na ] and [Cl⁻] were followed by acidosis. Blood pH was decreased by 0.13 unit at 6 hr, but partially restored by 24 hr. In the dace acclimated to neutral stream water, chloride cells were scattered in the gill filament. Following transfer to acid lake water, however, well-developed chloride cells were arranged in a radial fashion, forming follicular or gland-like structures in the gills. Each chloride cell was equipped with an apical pit, which faced a common lumen leading to the external environment. These findings provide morphological evidence for a significant role(s) of gill chloride cells in ion and acid-base regulation in the acid-tolerant dace.