The management of bilingualism in the Spanish autonomous community of Navarre is a source of tension. The implementation of English medium of instruction in the public educational system has clashed with attempts to break with the linguistic territoriality regime by promoting Basque schooling. This paper brings together ideologies on English and minority languages and explores how political practice is intertwined with language policy and planning and language ideology. More specifically, it examines the institutionalization of language ideologies through language policy-making in education and, particularly, through medium of instruction. The paper begins with a description of the bilingual regime in Navarre and an examination of how ideologies have shaped and legitimized language policy in education. It then moves on to an analysis of both Basque and English medium of instruction ideologies that inform policy-making. This paper shows that the dynamics introduced by multilingualism in education have had a reinforcing effect on previous language ideologies on bilingualism and, ultimately, have aggravated the language dispute. Finally, it discusses how medium of instruction serves as a terrain for language competition and as part of a broader struggle for language policy and institutional power.