Whaling in the Antarctic Sea on the admissibility of Japan's whale programme under the Whaling Convention highlights the importance of organ practice for the interpretation of the underlying treaty. Analysing the Court's reasoning against its earlier case-law, this article first assesses and affirms that plenary organ practice amounts to practice 'between the States' and, thus, to a subsequent ... [Show full abstract] agreement or subsequent practice within the meaning of Article 31(3)(a) and (b) of the Vienna Convention on the Law of Treaties (VCLT). It then assesses and denies that this goes with a special rule on subsequent practice to the effect of lowering the requirement for an agreement within treaty organs. While within organs silence is easily taken for a tacit agreement, this cannot overcome dissent. And this holds true with regard to organs within general treaty regimes as well as organs of international organizations with legal personality. Whereas the Court therefore rightly rejected the resolution calling for a proportionality test on lethal sampling as subsequent practice under Article 31 of the VCLT because it was not adopted by consensus, the Court is criticized for relying on resolutions of the International Whaling Commission (IWC) by way of a duty of cooperation to give due regard to organ practice. Instead, the more established category of other confirmatory practice pursuant to Article 32 of the VCLT is introduced, which would have permitted the Court to explicitly buttress its affirmation on a proportionality test by relying on a resolution still reflecting the view of a considerable majority of states parties. © 2017. Oxford University Press on behalf of EJIL Ltd. All rights reserved.