There is a broad consensus that native speakers' knowledge of the grammatical system of their language is predominantly implicit, i.e., unconscious and acquired incidentally rather than intentionally. Word meaning, in contrast, is regarded as the paradigm case of explicit, or declarative, knowledge (although some aspects of lexical knowledge, e. g. collocations and grammatical features, may be implicit). This paper presents evidence that knowledge of word meanings can also be implicit. 63 undergraduate students were given a self-evaluation task in which they were asked to assess their own knowledge of low frequency words, followed by a multiple-choice test providing an objective measure of their knowledge of the same words; they were asked to guess if they did not know the meaning of a word. Results indicate that even when participants claimed to be guessing, their performance was significantly above chance, indicating the existence of implicit knowledge by the guessing criterion (Dienes 2008).