This research consists of a longitudinal study of 12 female elementary preservice teachers' conceptual understanding over the course of several months. The context in which the participants received instruction was in an inquiry-based physics course, and the targeted science content was the cause of moon phases. Qualitative research methods, including observations and interviews, were used to investigate and describe participants' conceptual understanding over time. Participants were interviewed on their understanding of the cause of moon phases before instruction, 3 weeks after instruction, and again in delayed post-interviews several months after instruction. Patterns and themes in the participants' conceptual understanding were identified through constant-comparative data analysis. Consistent with results reported earlier, participants who had instruction that included recording and analyzing moon observations over time and psychomotor modeling of changes in moon phases were very likely to hold a scientific conceptual understanding shortly after instruction. The present study indicates a majority of participants continued to hold a scientific understanding six months or more after instruction. However, some participants reverted to alternative conceptions they had shown during the pre-interview. These results are interpreted utilizing contemporary conceptual change theory.