Research on the effect of bilingualism on third language acquisition was initially set in the framework of the study of potential differences between monolinguals acquiring a second language and bilinguals learning a third language. However, many authors have already started to concentrate on third language acquisition itself, and have investigated if there are any differences with regard to target language competence as a function of certain characteristics of the bilingual learner. The number of specific studies on the acquisition of phonology in a third language is extremely limited, though. The article presented here reports a research study on the phonological acquisition of English as a foreign language in an institutional context and, specifically, examines the effect of different levels of bilingual proficiency on phonological competence in the third language.