Socio-scientific issues in class have been proposed in an effort to democratise science in society. A micro-ethnographic approach has been used to explore how students elaborate arguments on a socio-scientific controversy in the context of small group discussions. Several processes of group argumentation have been identified. Students' arguments were elaborated from scientific data, common ideas and epistemological and strategic considerations. Students' social interactions influenced the patterns of argumentation elaborated within the group discussions. Implications of this study for the teaching of socio-scientific issues in class are discussed.