We examine whether and how teachers' major fields in college affect students' achievement, exploiting within-student variation across subfields in natural science (i.e. physics, chemistry, biology, and Earth science). Using middle-school students' data from the TIMSS and controlling for student-teacher fixed effects, we find that teachers improve students' achievement in subfields of natural sciences correspond to their college majors. Teaching practices explain about half of the effect, mostly accounted for by teachers' preparation for teaching science topics. The results are robust to potential endogenous matching between students and teachers.