As linguists develop a deeper understanding of the properties of individual varieties of speech, they often find it necessary to reclassify dialects as independent languages, based on the criterion of intelligibility. This criterion is applied here to Jejueo, the traditional variety of speech used on Jeju Island, a province of the Republic of Korea. Although Jejueo has long been classified as a nonstandard dialect of Korean, evidence from an intelligibility experiment shows that it is not comprehensible to monolingual speakers of Korean and therefore should be treated as a separate language, in accordance with the usual practice within linguistics. This finding calls for a revision to the standard language map of Kore