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Sperm whales are notable squid-eaters. They feed mainly on medium to largesized cephalopods at midwater levels and defecate near the surface. This suggests the existence of an upward transport of 8°Co by sperm whales from the mesopelagic zone (150-1,200 m). To elucidate this squid-whale route for this artificial radionuclide, 6°Co content was determined in squid and in predator whales captured by commercial whaling. In the Cephalopoda livers 8°Co levels of 30-500 mBq kg -~ wet were found and in the viscera of Odontoceti (toothed whales)15-40 mBqkg -~ wet. About 0.3 % of 8°Co ingested was estimated to be retained in a 23-year-old male sperm whale. In the livers of Bryde's whales, ~°Co levels of 40-80 mBqkg -~ wet were detected, but not in euphausiids and sardines, their possible prey. The level of Co in sperm whales was nearly the same as in Bryde's whales. Specific radioactivity 6°Co/SgCo in mBq pg-i was several times higher in sperm whale (1.1-1.6) than in cephalopods (0.19-0.77). Eating prey with a high content of 6°Co in the 1960's may have contributed to the present body burden in sperm whales with a long-life span. However, the origin of 8°Co in Bryde's whales is unknown.