Mock fiction is a genre of humorous, fictional narratives. It is pervasive in ado-lescents' peer-group interaction. Building on a corpus of informal peer-group interaction among 14 to 17 year-old German adolescents, it is shown how mock fiction is used to sanction identity-claims of peer-group co-members that are taken to be inadequate by the teller of a mock fiction. Mock fiction exposes and ridicules those claims by fictional exaggeration. Mock fiction is an indirect, yet sometimes even highly abusive means for criticizing and negotiating identities and statuses of peer-group members. The analysis shows how mock fiction is collaboratively produced, how it is used to convey criticism and to negotiate social norms indirectly, and how, in addition, it allows for performative self-po-sitioning of the tellers as skilled, entertaining tellers and socio-psychological diagnosticians.