Even if the topic of cross-linguistic influence in Third Language (L3) Acquisition has received increasing attention in the last years, results are still inconclusive as it is very difficult to isolate one single factor which determines cross-linguistic influence (García Mayo, 2012). In addition, recent studies have concluded that apart from the factors that normally modulate this phenomenon in L3 acquisition (psychotypology, age, or proficiency, among others), type of language teaching programs such as Content and Language Integrated Learning (CLIL) may also have an effect (Martínez Adrián & Gutiérrez Mangado, 2015).In this respect, most of the studies that have tackled first language (L1) use in CLIL settings have compared CLIL to non-CLIL learners (i.e. Celaya & Ruiz de Zarobe, 2010). However, few (pseudo)longitudinal studies have been conducted (i.e. Lázaro Ibarrola & García Mayo, 2012). In addition, to our knowledge, there is a lack of (pseudo)longitudinal studies addressing a correlation between the use of previously known languages during oral production and general proficiency as well as receptive vocabulary measures.This paper will try to fill this gap by examining the development of L1 use in an oral narration task together with general proficiency (Oxford Placement Test) and knowledge of receptive vocabulary (Vocabulary Level Tests) in two different age CLIL groups.Data were gathered from Basque/Spanish bilinguals learning L3 English in a CLIL setting. Both groups started learning English at the age of 3 but differed in length and amount of exposure. Results showed that older learners outperformed younger learners in general proficiency and vocabulary knowledge. However, both groups behaved in the same way in terms of L1 use except for the production of foreignizings, which was more common among the learners with lower proficiency. Statistical analyses revealed significant correlations between general proficiency, receptive vocabulary and L1 use in both groups.