The implementation of the Mother Tongue-Based Multilingual Education (MTB-MLE) policy in the Philippine basic education has gained traction among educators. Skepticism as to its effectiveness is evident especially in mathematics class where English is the universally recognised medium. This study explored how elementary teachers from a rural school in the Philippines appropriate the MTB-MLE policy in a mathematics classroom. Their attitudes toward the policy and the challenges met in the implementation were examined. An instrumental case study was conducted with documentary analysis, non-participant observation, in-depth interview, and focus group discussion as methods of data collection. Five Grades 1-3 teachers served as participants. Results showed that teachers do not fully observe the policy. They code-switch during discussion and do not religiously use the prescribed teaching guide. Fidelity to the policy is only apparent in the presence of authority. Teachers generally have negative attitudes towards the policy which is due to their colonial thinking, perceived complexity, and non-utilitarian view of the local language. Mismatch in the students' language and the language of instruction, lack of equivalent local terms for some mathematics terms, and haphazardly done teaching and learning materials are the challenges that impede teachers' effective implementation of the policy.