This paper challenges the view that attitudes towards genetically modified (GM) crops in agriculture are "utterly resistant to persuasion", as Scholderer (2005) humorously suggests in a review of the literature. Methodologically, this is achieved by studying opinions as they emerge in situated interaction (Myers 2004), in casu in focus groups interviews with both GM-experts and lay persons without specific knowledge on GM-crops. The paper analyses the lay persons' responses to persuasive expert utterances as inventive contributions to the discussion, not just as reactions showing either support or rejection. That is, the paper analyses the topoi, the argumentative ‘places', realized by the lay persons in dealing with and making sense of the new knowledge presented by the experts. Finally, the paper identifies the social identities as participants in a public debate, which are enacted by the lay contributions.