This empirical investigation compares the ability of adult multilingual learners of English to perform a metalinguistic task involving paying close attention to meaning and/or form in target utterances/sentences varying in semantic appropriateness and grammatical correctness. In a listening task, adult multilingual learners of English heard these utterances in a constructed short story and then judged their grammatical correctness and semantic appropriateness under time pressure. They then performed a written "post hoc" grammatical judgement and correction task on the same sentences at their own pace. Our first hypothesis was that these multilingual learners would a "priori" be highly metalinguistically aware, and thus adept at sifting formal grammatical information from semantic information both online as they heard the items, as well as "post hoc" in the written task. We also expected that lesser as well as more experienced multilingual participants would be equally proficient at analysing the grammatical constructions in the "post hoc" written task. Both of these assumptions were borne out by results. The results are relevant to the debate about metalinguistic advantages for adult foreign language learners who are not merely bilingual, but are at least trilingual, and how these advantages are employed during meaning and form processing in a foreign language. (Contains 2 tables and 4 figures.)