The research on first language learning is the premise for the Philippines Mother-Tongue-based Multi-Lingual Education (MTB-MLE) Policy which requires schools to deliver the Kindergarten to Grade 3 curriculum in the mother tongue (home language) of the school’s community (Corder, 1983; Walter & Dekker, 2011. Salmona, 2014; Yadav, 2014). Considering that the national curriculum requires children to transition to and learn in Filipino and English at the start of Grade 4, the question of mother tongue’s effect on second and third language acquisition is not academic, but central to the policy debate on MTB-MLE efficacy. The MTB-MLE policy consists of five discrete areas: curriculum, learning resources, assessment, teacher recruitment and training, and community support. MTB-MLE has proven challenging with respect to the myriad languages and dialects. Out of the 180 plus languages spoken, only 19 have been formally supported with an official orthography, standard learning resources and teacher professional development materials. In communities with non-supported languages, teachers contextualize the teaching and learning materials, often translating and adapting from the linguistically nearest mother tongue to their own. In a 2019 study on MTB-MLE, the Philippines Institute of Development Studies found inconsistent implementation across schools, including teachers’ negative attitudes toward MTB-MLE, linguistic diversity of learners and classrooms, and lack of teaching and learning materials being key factors hindering its implementation (PIDS, 2019). The Bicol Region poses a particular challenge. In an area slightly larger than Connecticut with a population of just under 4 million, thirteen different languages plus numerous dialects are spoken in the various provinces, cities and towns that dot this volcanic region (Lobel, 2019). The standard language of Central Bikol, which is the mother tongue of approximately half the population consists of six different local dialects depending on the locale. Wedged in the middle of the Central Bikol-speaking area is a cluster of five distinct languages: Rinconada, Buhi-non, Bikol Libon, West Albay Bikol, and Miraya, with only one or two municipal communities each that speak these languages. Under the USAID Advancing Basic Education in the Philippines (ABC+), RTI conducted a longitudinal study that provides new evidence on the efficacy of MTB-MLE. The study tracked the language and literacy acquisition of four groups of learners: those Central Bikol learners who are learning in a fully supported language; Buhi-non speaking learners who are learning in an unsupported language (ie, Buhi-non); Central Bikol speakers who’s language of instruction is Tagalog and Tagalog learners who’s language of instruction is Tagalog. The findings show evidence that the Central Bikol learners whose LOI is Tagalog are performing at par or worse in nearly all domains of reading in their first (Central Bikol), second (Tagalog/Filipino) and third languages (English). The findings show a flattening of their trajectory in terms of the pace of language and literacy acquisition, as well as significant equity gaps in comparison to their Tagalog peers. The evidence points toward continued support to MTB-MLE, despite the challenges in implementation.