There is evidence that science teachers give naive responses to direct questions about the nature of science. However, there is also evidence that such responses underpredict the more sophisticated knowledge that teachers may use in classroom situations. The purpose of this study was to characterize the informal ideas used by teachers in situations directly relevant to their teaching of science. The sample comprised 50 Mexican primary school teachers. Three areas of informal expertise were addressed: scientists and their work; scientific inquiry; and data measurement. The teachers' thinking was explored through questionnaires and semistructured interviews using pedagogically relevant contexts. The database was analyzed first to describe ideas shared by teachers and second to identify any recurrent themes and patterns among responses. Teachers' responses were characterized around four areas of discourse: demarcation of science; scientific procedures; approaches to reliable knowledge; and professional and institutional features of science. The teachers' responses were diverse in their "contextualization," that is, they incorporated specific background and contextual details to a different extent. Most responses showed limited or intermediate contextualization across all four areas of discourse, though some more sophisticated responses were noted. A general framework was developed to characterize this diversity in teachers' responses. This paper concludes with a discussion of the extent to which this sample of teachers was equipped to discuss the nature of science in pedagogical contexts and, given their starting points, how they might be supported in developing their expertise in doing this. Implications for teacher education and curriculum development are presented. (Contains 2 tables.)