The use of previously known languages (PKL) during task-based interaction has been the focus of attention of numerous English as a second Language and English as a foreign language investigations, predominantly with adult learners. These studies have tackled factors such as proficiency, onset age, gender, task-type (i.e. Alegría de la Colina & García Mayo, 2009; Muñoz, 2007), and to a lesser extent, task-modality (Azkarai & García Mayo, 2015; Payant & Kim, 2019).More recent studies have examined CLIL learners and have mainly explored proficiency effects (i.e. Arratibel-Irazusta & Martínez-Adrián, 2018; Pladevall & Vraciu, 2017). However, there is an acute need to investigate other variables, such as task-type and task-modality, especially in the case of young learners.This article will contribute to the scarcity of studies on the interface between task-modality and the use of PKL by examining the resort to Basque/Spanish during the oral interaction of young CLIL learners (aged 10-11) while performing two collaborative tasks in English: a speaking task and a speaking+writing task. Participants started learning English in pre-primary education and enrolled in a CLIL programme at the age of 8. They were considered beginner learners according to the proficiency level test administered at the outset of the study. Children were paired up into 24 same-proficiency dyads to perform both tasks. Even if these tasks are considered information-gap tasks, the speaking+writing task requires the submission of a written product. All PKL turns were identified and subdivided into predominant or minor PKL turns. Subsequently, they were classified according to the functions they served on the basis of Azkarai and García Mayo (2015) and Storch and Aldosari (2010), namely off-task, metacognitive talk, grammar talk, vocabulary, phatics, and mechanics.The examination of the results indicated that these young learners were extensive users of their PKL, and in particular of predominant PKL turns. Task-modality was particularly evident in the case of amount of PKL use, asa higher number of PKL turns were obtained in the speaking+writing task. Task-modality had a limited effect on the functions of PKL use though,which contrasts with previous studies with adults. In the present study, a higher use of PKL was observed in metacognitive talk and mechanics in the speaking+writing task, while no differences emerged between tasks in the other categories. Unlike previousresearch with adults in which vocabulary was more common in the speaking task, in the present study vocabulary was key in both tasks to move them forward.Likewise, while grammar talk was more frequent in the speaking+writing task in adults, grammar talk mediated by PKL was nearly inexistent in the database, a finding that could be ascribed to young children’s low number of elaborations in the form of metalinguistic explanations. All in all, despite the extensive use of their PKL, these young and low-proficient learners employ them as cognitive tools that facilitate the organization of the tasks, the co-construction of meaning and the attention to formal aspects of language such as mechanics.