This paper explores new elementary teachers' instructional representations and how these are related to their science subject matter knowledge. One pair of prospective elementary teachers studied here exhibited a well-integrated, principled, and scientifically accurate understanding of the science they were teaching. The other pair exhibited less scientifically accurate and integrated knowledge. The pair with stronger subject matter knowledge developed instructional representations that were more scientifically and pedagogically appropriate. A perspective on one aspect of pedagogical content knowledge--knowledge of instructional representations--is presented. Real-world applications are hypothesized to play a crucial mediating role for elementary teachers. The paper concludes with a discussion of implications for elementary science teacher educators and researchers, including the importance of attending to how prospective teachers apply science knowledge to real-world situations. Parts of this work were presented at the 2000 AERA and NARST meetings.