This study examines pronunciation errors made by Filipino learners of BIPA from an interlanguage phonology perspective. The study aims to identify common pronunciation errors, explain the influence of the learners’ first and second language (L1-L2): Tagalog and English, and describe how these errors reflect their interlanguage development. The participants were 10 BIPA learners from the Polytechnic University of the Philippines. Data were collected through classroom observation and audio recordings during speaking activities. The pronunciations were analyzed phonologically by identifying and categorizing pronunciation errors. The findings show that most errors involved vowel substitutions, consonant changes, and difficulties with consonant clusters and nasal sounds. Most errors involved vowel substitutions, especially changes between /a/, /ə/, /e/, /u/, and /o/. Learners also had difficulties in pronouncing some Indonesian consonants such as /h/, /ŋ/, and /c/. In several cases, learners omitted sounds or added extra sounds to make pronunciation easier. These patterns indicate that their L1-L2 give strong influence toward their learning process in BIPA class. It also demonstrates learners’ adaptation to Indonesian phonology. This research suggests that BIPA teachers should be more focus on pronunciation practices with perception-based training, especially on Indonesian vowel contrasts and consonant sounds that are difficult for BIPA learners in the Philippines.