DPs such as the tallest boy, whose head noun is preceded by a superlative adjective, and DPs such as the only boy, whose head noun is preceded by only, are definite in the morpho-syntactic sense. But as is well-known at least since Higgins (1973), they can sometimes be semantically non-definite. We account for this optional non-definiteness within the theory of superlatives proposed in Heim (1999), by the assumption that nouns take degree arguments just like gradable adjectives, and that only and -est are phonetic realizations of the same abstract morpheme (as proposed in Bhatt 2002, 2006).