Previous and current research identify Metalinguistic Awareness (MLA) as a possible cause of the positive effects of bilingualism on cognitive development and Third Language Acquisition (TLA). Although it has been acknowledged that MLA is strongly affected by literacy and grammar-related activities, only a few studies have focused on the context and method of acquisition of the bilingual learners' L2 to account for the mediating effect of MLA in TLA. This study tested the assumption that formal instruction in multiple languages enhances MLA in bilinguals which, in turn, facilitates the process of TLA. 42 bilinguals, with different levels of proficiency and MLA in German L2, were tested for their ability to learn an additional language through an artificial language task (Llama-F, Meara, P. 2005. Llama Language Aptitude Tests. Swansea. Lognostics). Correlations and multiple regression analyses were conducted to examine the relationship between bilinguals' level of explicit MLA and performance in the TLA task. The influence of various potential predictors was considered including explicit MLA; number of languages; overall proficiency and instruction in German L2. The results indicate that bilinguals with higher levels of explicit MLA also perform better in TLA, after controlling for the aforementioned variables in the model. The findings are discussed in pedagogical terms, suggesting that learners should be stimulated and assisted in the process of conscious reflection and manipulation of the language system(s) and the learning strategies developed in previously learned languages, to obtain a positive outcome in TLA.