Teacher knowledge of science content is an important but under-studied construct. A curricular and professional development intervention consisting of a fifth grade science curriculum, teacher workshops, and school site support was studied to determine its effect on teachers' science content knowledge as measured by a science knowledge test, a questionnaire, and classroom observations. These three measures, along with college science courses taken, were then used to examine the effect of teachers' science content knowledge on student achievement outcomes. The intervention had a significant effect on the treatment group teachers' science knowledge test scores and questionnaire responses compared to the control group, but not on the classroom observation ratings. Teachers' scores on the science knowledge test were found to be the largest significant teacher-level predictor of student achievement outcomes regardless of participation in the intervention.