This study focused on the conceptual understandings held by 78 preservice elementary teachers about moon phases, before and after instruction. Participants in the physics groups received instruction on moon phases in an inquiry-based physics course; participants in the methods group received no instruction on moon phases. The instructive effect of two different types of preinstruction interviews also was compared. The instruction on moon phases used in the study is from Physics by Inquiry by Lillian McDermott. In the study, the method of inquiry followed a qualitative design, involving classroom observations, document analysis, and structured interviews. Inductive data analysis identified patterns and themes in the participants' conceptual understanding. Results indicate that without the instruction, most preservice teachers were likely to hold alternative conceptions on the cause of moon phases. Participants who had the instruction were much more likely to hold a scientific understanding after instruction. The instruction appears to be more effective in promoting a scientific understanding of moon phases than instruction previously reported in the literature. It also appears that using a three-dimensional model or making two-dimensional drawings during the preinstruction interviews does not have instructive value.