ABSTRACT: We measured white muscle isotopic compositions and aspect ratios (ARs) of caudal fins in juvenile Pacific bluefin tuna Thunnus orientalis (PBT) to examine ontogenetic dietary shifts and swimming ability. ARs averaged 3.9 in fish with fork length (FL) 15 cm, 4.6 in fish with FL of 30-35 cm, and 6.7 in adult fish with FL >200 cm. Mean isotope values of carbon and nitrogen in white muscle increased from -18.3 to -18.0‰ and from +7.2 to 8.9‰ in ~20 cm fish to -16.5‰ and +12.3‰ in 30-35 cm fish, respectively, with a shift at 25 cm FL. This shift was much earlier than that reported for yellowfin tuna T. albacares (40-50 cm FL). Our results suggest that, after moving to Japanese coastal areas and reaching a FL of 25 cm, PBT rapidly develop swimming abilities by achieving endothermy and switching from a diet of small squid and zooplankton to a diet based on fish prey items that have greater body mass with higher calorific content and swimming ability. This may give them a selective advantage and allow high energetic expenditure.