The purpose of this study was to delineate the factors that mediate the translation of preservice teachers' conceptions of the nature of science (NOS) into instructional planning and classroom practice. Fourteen preservice secondary science teachers participated in the study. Prior to their student teaching, participants responded to an open-ended questionnaire designed to assess their conceptions of the NOS. Analysis of the questionnaires was postponed until after the completion of student teaching to avoid biasing the collection and/or analysis of other data sources. Throughout student teaching, participants' daily lesson plans, classroom videotapes, and portfolios, and supervisors' weekly clinical observation notes were collated. These data were searched for explicit references to the NOS. Following student teaching, participants were individually interviewed to validate their responses to the open-ended questionnaire and to identify the factors or constraints that mediate the translation of their conceptions of the NOS into their classroom teaching. Participants were found to possess adequate understandings of several important aspects of the NOS including the empirical and tentative nature of science, the distinction between observation and inference, and the role of subjectivity and creativity in science. Many claimed to have taught the NOS through science-based activities. However, data analyses revealed that explicit references to the NOS were rare in their planning and instruction. Participants articulated several factors for this lack of attention to the NOS. These included viewing the NOS as less significant than other instructional outcomes, preoccupation with classroom management and routine chores, discomfort with their own understandings of the NOS, the lack of resources and experience for teaching the NOS, cooperating teachers' imposed restraints, and the lack of planning time. In addition to these volunteered constraints, the data revealed others related to an intricate interaction between participants' perspectives on the NOS, pedagogy, and instructional outcomes. © 1998 John Wiley & Sons, Inc. Sci Ed82:417–436, 1998.