While teacher educators have had some success in helping prospective teachers understand the nature of science (NOS), they have been less effective in helping prospective teachers teach NOS. Though several studies have alluded to impacts of various interventions on developing pedagogical content knowledge (PCK) for NOS, the nature, source, and development of PCK for NOS has not yet been investigated in any systematic way. This study addresses that gap by identifying critical incidents in the development of PCK for NOS of a prospective elementary teacher as she transitions from her methods course to student teaching. Analysis of data collected over a 2-year period illuminates pedagogical dilemmas faced by the prospective teacher in enacting NOS instruction within a school culture of primarily "traditional" science teaching. Through construction of critical incident vignettes, narrative inquiry is employed to illustrate the changes in her PCK for NOS over time, and the experiences that facilitated these changes. Implications for supporting the development of PCK for NOS within teacher education are discussed.