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To understand the migratory behavior and habitat use of the Japanese eel Anguilla japonica in the Kaoping River, SW Taiwan, the temporal changes of strontium (Sr) and calcium (Ca) contents in otoliths of the eels in combination with age data were examined by wavelength dispersive X-ray spectrometry with an electron probe microanalyzer. Ages of the eel were determined by the annulus mark in their otolith. The pattern of the Sr: Ca ratios in the otoliths, before the elver stage, was similar among all specimens. Post-elver stage Sr: Ca ratios indicated that the eels experienced different salinity histories in their growth phase yellow stage. The mean (±SD) Sr: Ca ratios in otoliths beyond elver check of the 6 yellow eels from the freshwater middle reach were 1.8±0.2 x 10-3 with a maximum value of 3.73 x 10-3. Sr: Ca ratios of less than 4 x 10-3 were used to discriminate the freshwater from seawater resident eels. Eels …