This study examines the relationship between teacher knowledge and student learning for 9,556 students of 181 middle school physical science teachers. Assessment instruments based on the National Science Education Standards with 20 items in common were administered several times during the school year to both students and their teachers. For items that had a very popular wrong answer, the teachers who could identify this misconception had larger classroom gains, much larger than if the teachers knew only the correct answer. On items on which students did not exhibit misconceptions, teacher subject matter knowledge alone accounted for higher student gains. This finding suggests that a teacher's ability to identify students' most common wrong answer on multiple-choice items, a form of pedagogical content knowledge, is an additional measure of science teacher competence.