The Banyumasan dialect or better known as "Ngapak" is part of the Javanese dialect which has striking phonological characteristics when compared to the Mataraman dialect. This study aims to describe the form of phoneme variation in the Banyumasan dialect spoken in two regions, namely Banyumas Regency and Cilacap Regency. This research was conducted using a qualitative descriptive method through a dialectological approach, focusing on phoneme units as the smallest unit in the language sound system. The results of field observations and direct interaction with native speakers from Banyumas and Cilacap show that there are several forms of phonemic variation, including: (1) apokope, which is the elimination of vowel sounds at the end of words, such as in the word "koe" which becomes "ko" or "uwis" becomes "wis"; (2) the addition of phonemes, especially epenetic vowels that are added to refine articulation, such as in the word "arep" which becomes "arepan"; and (3) consonant substitution, e.g. simplification of the phoneme /ai/ which is reduced to /i/ in the word "wae" to "baen", or the change of the phoneme /n/ to /ɲ/ (ng) in the phrase "nang endi" which changes to "ngendi". This phonemic analysis shows that each change has a certain pattern that reflects systemic consistency in people's spoken language. This not only reflects linguistic adaptations to local communication needs, but also demonstrates the social function of language variation as part of community identity. In other words, the phonemic differences found are not just deviations from the standard form, but a reflection of the typical phonological structure that develops naturally in the Banyumasan language community. This research also strengthens the understanding of regional language dynamics and the importance of preserving local languages in the midst of globalization.