In 2009 the Philippines introduced a mother tongue-based multilingual education language policy requiring the “mother tongue” as the language of instruction (LOI) in kindergarten through grade 3. Using teacher classroom language data collected from four LOI groups in 2019, we compared the frequency of teachers’ use of the target LOI in different contexts, including urban versus rural classrooms, classrooms with relatively homogeneous student language backgrounds versus more heterogeneous classrooms, and classrooms with materials in the target language versus classrooms without. We also examined language usage against characteristics of the teacher populations, including language background, years of experience, training, and beliefs about the best language for initial literacy. The results strongly suggest that the most influential levers for increasing teacher usage of a designated LOI in these contexts are ensuring that teachers are assigned to schools where the LOI matches their own first language and providing teaching and learning materials in the target LOI, especially teacher’s guides. These two factors were more strongly and more consistently correlated with teacher use of the LOI than all other variables examined. The linguistic homogeneity of the student population also showed a statistically significant though lower impact on teacher language usage. This document was developed with support from the American people through the United States Agency for International Development.