The L1 subject is a central meeting place for all students regardless of their linguistic backgrounds. Thus explorations of multilingualism in the L1 subject provide the potential for enhancing all students' multilinguality. In Norway, several policy papers have emphasised the important role of the L1 Norwegian subject in promoting students' multilingualism as a resource, but guidelines are lacking on how this should be facilitated. Consequently, L1 teachers' beliefs about multilingualism are central to understanding how and to what extent they implement a multilingual pedagogy. Whereas previous research in the Norwegian context has mainly explored foreign language teachers' beliefs on multilingualism, this study investigated L1 Norwegian teachers' beliefs on how the subject can be taught in an increasingly diverse language classroom. Ten upper secondary school teachers discussed these topics in focus groups. Most of the teachers had a language-as-problem orientation to their students' multilingualism. In particular, they found it challenging to improve minority students' Norwegian skills. Moreover, they rarely encouraged the use of minority students' multilingualism as a resource in the classroom. They also reported little guidance from textbooks. The findings suggest a strong need for supporting L1 subject teachers in developing a multilingual mindset and for creating suitable teaching materials.