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In the summer of 2013, we measured the radiocesium (134Cs and 137Cs) concentrations in aquatic organisms inhabiting the estuary of the Abukuma River, which flows through the Fukushima and Miyagi Prefectures. Radiocesium concentrations in four muscle-tissue samples of blackhead seabream ( Acanthopagrus schlegeli) [1240, 914, 202 and 106 Bq kg−1 wet weight (w.w.)] and two whole-body samples deriving from multiple individuals of flathead grey mullet ( Mugil cephalus) (129 and 110 Bq kg−1 w.w.) exceeded the Japanese standard limit for radiocesium levels in foods (100 Bq kg−1 w.w.). However, radiocesium concentrations in the other fish samples were lower than the Japanese standard limit. In addition, radiocesium concentrations in crustaceans, major prey items of omnivorous and carnivorous fish species in the Abukuma River estuary, were generally low (0.444-15.1 Bq kg−1 w.w.). Stable isotope analysis indicated that the feeding habit of the three heavily contaminated blackhead seabream, which the radiocesium concentrations were about several times or more higher than the Japanese standard limit, were similar to those of other individuals collected from the same sampling site. Therefore, the difference in the radiocesium level between the three heavily contaminated individuals and the less-contaminated individuals of blackhead seabream was not because of a recent feeding habit in the Abukuma River estuary. Environmental conditions in the Abukuma River estuary in the summer of 2013 would not have had the potential to increase radiocesium concentrations in the muscle tissues of fish inhabiting the estuary to levels greater than 100 Bq kg−1 w.w. [ABSTRACT FROM AUTHOR]