Although studies repeatedly show both the importance of teachers' science subject matter knowledge (SMK) and the fact that teachers have limited SMK, there are few opportunities for teachers to formally enhance their understanding of science after entering the classroom. Therefore, there is significant need for teachers to develop their SMK through classroom experience. In this longitudinal study, we explore the development of new secondary science teachers' SMK by analyzing concept maps constructed in their 1st year of teaching and in their 5th year of teaching. We present a scoring system that reliably accounts for both the quality of connections and knowledge structure. Based on the findings presented, we conclude that these new chemistry teachers' SMK did not change significantly from their 1st year of teaching to their 5th year of teaching. Implications for teacher educators and future research are presented.