Using data collected from two multiyear teacher professional development projects employing randomized control trials, this study describes the development and validation of a paper-based test of elementary teachers' science content knowledge (SCK). Evidence of construct validity is presented, including evidence on internal structural features using Rasch measurement models. Results from 183 treatment group and 176 control group teachers from Project 1 demonstrated that the SCK test had acceptable person reliability at baseline; at later time points the test was easy for the teachers and person reliability was below acceptable. Results from Project 1 informed changes made to the test for use in Project 2, including an increase in the difficulty level and the development of two equated forms. Results from the 148 treatment and 139 control teachers from Project 2 demonstrated that the test had acceptable reliability across two time points and was a better match to teachers' SCK.