A strategy-based reading promotion program was implemented over a course of 8 months in 65 classes from grades 5 to 7 (student age M?=?11.2 years, SD?=?1.0). It was investigated whether teachers flexibly adapted their pedagogical content beliefs (PCBs) on the teaching of reading according to the instructional principles of the intervention. Fifteen teachers demonstrated the assumed PCB development curves, that is, an initial increase of direct-transmissive positions followed by an increase in the endorsement of constructivist beliefs. Students of teachers whose PCBs showed this belief development pattern demonstrated significantly larger reading achievement gains than students of the other teachers. Thus, teachers' PCBs are--in principle--open to change which can turn out relevant for student achievement gains.