Following recent FAO recognition that what were thought separate Atlantic and Pacific stocks of Thunnus thynnus are actually distinct species, this article examines the considerable variation in how tunas are named and classified in the UN Convention on the Law of the Sea and other fisheries treaties. Two consequences related to the superimposition of the UN Fish Stocks Agreement on this variation are highlighted. The author concludes that, until FAO names gain wider use, it is the Latin scientific names rather than the common names in the “authentic” vernacular treaty texts on which reliance should be placed for legal purposes.