This study investigated the influence of an explicit and reflective inquiry-oriented compared with an implicit inquiry-oriented instructional approach on sixth graders' understandings of nature of science (NOS). The study emphasized the tentative, empirical, inferential, and imaginative and creative NOS. Participants were 62 sixth-grade students in two intact groups. The intervention or explicit group was engaged in inquiry activities followed by reflective discussions of the target NOS aspects. The comparison or implicit group was engaged in the same inquiry activities. However, these latter activities included no explicit references to or discussion of any NOS aspects. Engagement time was balanced for both groups. An open-ended questionnaire in conjunction with semistructured interviews was used to assess participants' NOS views before and at the conclusion of the intervention, which spanned 2.5 months. Before the intervention, the majority of participants in both groups held naive views of the target NOS aspects. The views of the implicit group participants were not different at the conclusion of the study. By comparison, substantially more participants in the explicit group articulated more informed views of one or more of the target NOS aspects. Thus, an explicit and reflective inquiry-oriented approach was more effective than an implicit inquiry-oriented approach in promoting participants' NOS conceptions. These results do not support the intuitively appealing assumption that students would automatically learn about NOS through engagement in science-based inquiry activities. Developing informed conceptions of NOS is a cognitive instructional outcome that requires an explicit and reflective instructional approach. © 2002 Wiley Periodicals, Inc. J Res Sci Teach 39: 551–578, 2002