Even if the topic of cross-linguistic influence in Third Language (L3) Acquisition has received increasing attention in the last years and its study has tried to identify the specific conditions in which this phenomenon takes place, results are still inconclusive as it is very difficult to isolate one single factor which determines cross-linguistic influence in L3 acquisition (García Mayo 2012). In addition, recent studies have concluded that apart from the factors that normally determine this phenomenon (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, in press).Most of the studies that have tackled first language (L1) use in CLIL have compared CLIL to NON-CLIL learners (i.e. Celaya & Ruiz de Zarobe, 2010; Martínez Adrián & Gutiérrez Mangado, in press). However, few (pseudo)longitudinal studies have been conducted in this respect (i.e. Lázaro Ibarrola & García Mayo, 2012). In addition, to the knowledge of the authors, there is a lack of (pseudo)longitudinal studies addressing a correlation between the use of previously known languages during 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 (interactional strategies, transfer lapses, code-switching and discourse markers) in an oral narration task together with general proficiency and knowledge of receptive vocabulary in two different age CLIL groups.Participants are Basque/Spanish bilinguals learning L3 English in a CLIL setting. Both groups started learning English at the age of 3 but differ in length and amount of exposure. At testing time, the first group (n=28) was attending 2nd grade of secondary education and had received 1887 of hours of exposure, while the second group (n=20) was in the 4th year of secondary education and had received 2057 hours. Students were first administered the standardized Oxford Quick Placement Test and the 2,000 word frequency band from the receptive version of the Vocabulary Level Test (VLT). Subsequently, they were asked to narrate the well-known story “Frog, where are you?” with visual support provided by a series of vignettes.Results showed that students seemed to be developing towards the target language, as older learners outperformed younger learners in both written tests (the Quick Placement Test and the 2,000 Vocabulary Level Test). However, transfer effects were still present in their oral productions in both age-groups, as only foreignizings showed a significant decrease from the younger group to the older group, contradicting previous studies that have found less cross-linguistic influence in more proficient learners (Martínez Adrián & Gutiérrez Mangado, in press). These findings seem to indicate that the difference in exposure time between both CLIL groups enhances the development of general proficiency and receptive vocabulary, but does not promote a minimization of L1 effects.