This study explores socio-cognitive functions of mother tongue use during group work of bilingual pupils (Turkish-Dutch) in two mainstream primary schools in Flanders (Belgium). In each school, a group of four children performed three different tasks, related to different subjects of the curriculum in order to explore functions of mother tongue use (i.e. Turkish). During task performance, both groups of four bilingual pupils were guided by the first author of this study instead of their regular teacher, in a separate classroom. Following school language policy, task instructions were given in one language only, Dutch, the prescribed medium of instruction at school. However, during task performance, the pupils were allowed to use whatever languages they chose. Our data showed that the mother tongue was used for a range of sociocognitive functions, including content-related and management-related talk. The main findings of this study thus indicate that the mother tongue fulfills an important role for cognitive as well as for socio-affective purposes, even for bilingual pupils who are not used to drawing on their full linguistic repertoire during task performance. The results challenge the view, currently prevalent in Flemish education, that a submersion approach is the only viable option for bilingual pupils speaking a minority language.