This study explored the process of physics teaching assistants' (TAs) PCK development in the context of teaching a new undergraduate introductory physics course. "Matter and Interactions" (M&I) has recently adopted a new introductory physics course that focuses on the application of a small number of fundamental physical principles to the atomic and molecular nature of matter. The purpose of this study is to describe how the PCK of physics TAs in the "M&I" course was developed and enacted. Data collection began during the TA preparation workshop that was held before the semester and continued during the recitation and laboratory classes that the TAs taught during the semester. Through a qualitative, multiple case study research design, data were collected from multiple sources: non-participant observations, digitally recorded video, semi-structured interviews, TAs' written reflections, and researchers' field notes. Data analysis consisted of cross-case and within-case analyses. Data were analyzed using a constant comparative method. After data analysis, 2 main themes emerged related to the development of TA's PCK: (a) TAs' PCK developed from propositional knowledge to personal practical knowledge and (b) the process of knowledge development consisted of 3 phases: accepting, actualizing, and internalizing. The results of this study will not only contribute to the quest to identify the nature of professional knowledge of science teaching but also help guide preparation for innovative introductory physics courses and other college level science courses in the future.