The role of the first language (L1) in the learning of a second language (L2) has been widely studied as a source of cross-linguistic influence from the native system (Gass and Selinker, "Language Transfer in Language Learning," John Benjamins, 1992). Yet, this perspective provides no room for an understanding of language as a cognitive tool (Vygostsky, "Mind in Society: The Development of Higher Psychological Processes," Harvard University Press, 1978), that is, language as a mediating tool in all forms of higher-order mental processing. Recent findings in both foreign language classrooms (Anton and DiCamilla, "The Modern Language Journal" 83: 233-247, 1999; Brooks and Donato, "Hispania" 77: 262-274, 1994) and immersion classrooms (Swain and Lapkin, "Language Teaching Research" 4: 251-274, 2000) suggest that the L1 may be a useful tool for learning the L2. This line of research argues that an L1 shared by learners provides cognitive support that allows them to work at a higher level than that which would be possible if they were just using the L2. This paper reports the findings of a study which analyses the use of the L1 and its functions in the oral interaction of twelve pairs of undergraduate EFL learners with low proficiency in the target language while engaged in three collaborative tasks (jigsaw, text reconstruction and dictogloss). Our findings indicate that the L1 is an important tool for these learners and that there is task-related variation in its use.