Recent studies have concluded that type of language teaching programs such as Content and Language Integrated Learning (CLIL) may have an effect on the use of previously known languages. Most of the studies that have tackled the use of previously known languages in the CLIL literature have compared CLIL to non-CLIL learners. However, few (pseudo)longitudinal studies have been conducted in this respect. This paper will try to fill this gap by examining the use of previously known languages (L1 Spanish/L2 Basque) in an oral narration task in two different age/proficiency CLIL groups. Participants are Basque/Spanish bilinguals learning English as a third language (L3) in a CLIL setting. Both groups started learning English at the age of 3 but differ in length and amount of exposure. Results show that both groups behave in the same way in terms of the use of previously known languages except for the production of foreignizings, which were more frequent in the younger group. The examination of the results also revealed that discourse markers was the most common manifestation of use of previous linguistic experience when compared to the rest of the categories.