In English-medium instruction (EMI) classrooms in Japan, less proficient English speakers are often marginalized, positioned as powerless, and quiet (e.g., Iino, 2019). This situation is problematic for inequitable access to activity and the possible consequences for their identity and emotional wellbeing. However, few studies have examined how exclusion is co- constructed in such a context. This study addresses that lack, illustrating how exclusion is co- constructed by shedding light on the microlevel social interactions in an EMI classroom. In the study, observation notes, students’ weekly journals, and interview data were gathered. The findings show that fast-paced interactions and some gestures made the emergent bilinguals hesitant to participate in the discussions. Also, a mismatch of expectations from both the emergent bilinguals and the fluent speakers suggested that the exclusion was unintentional. An analysis of the findings underscores how social interactions, if not sensitive enough towards less proficient speakers, may cause unintentional exclusion and inflict emotional harm in the classroom.