the strontium (sr) and calcium (ca) concentrations in the otoliths of the skipjack tuna katsuwonus pelamis collected in the western pacific ocean were examined by wavelength dispersive x-ray spectrometry on an electron microprobe. otolith sr:ca ratios of the tunas collected off the marshall islands in the tropical waters were constant over the life history transect of the otolith. in contrast, ratios in most tunas collected off sanriku in the temperate waters in october fluctuated from low to high at 1100–1800 μm from the core. a similar fluctuation of otolith sr:ca ratio from low to high was found in a skipjack tuna that was tagged and released off sanriku, and then recaptured off the palau islands. therefore, at least two life history patterns of the skipjack tunas, of global migration toward northern temperate regions and local residence in tropical spawning grounds, are suggested by specimens collected in the western pacific ocean.