Indonesian Language for Foreign Speakers (BIPA) is an Indonesian language learning aimed at foreigners. The most challenging part for foreign speakers in BIPA learning is understanding the morphological processes. This study aimed to describe the prefix /me-/ used by foreign speakers in terms of productivity, allomorphy, and usage. This research was conducted at Yale University, USA, in 2022. The conclusions of this research were (1) The process of affixation of the prefix /me-/ produced six allomorphs; (2) There was the assimilation of the first letters /k/, /p/, /s/, and /t/ in the affixation process; (3) There were four forms that appeared in the form of single words, concatenated words, compound words, and repeated words; (4) There were twelve kinds of meanings produced; (5) There was allomorph /meng-/, which was not in accordance with the Indonesian language rules; and (6) The prefix /me-/ was omitted in everyday conversation in informal situations.