Abstract This study investigated the relationship between foreign language (FL) oral proficiency development and teachers’ pedagogical practices in content and language integrated learning (CLIL). A one-group pretest-posttest design was employed to analyze changes in Japanese university students’ FL proficiency over time, alongside classroom observations and interviews that captured their teachers’ classroom pedagogies in CLIL courses. The results generally indicated that students improved their capacity to achieve communicative goals but failed to exhibit progress in oral linguistic competence. The former outcome was attributed to predominantly meaning-focused instruction, the use of dialogic pedagogy, and ample opportunities to practice FL speaking. In contrast, the absence of explicit language instruction and limited practice of fluency development tasks was perceived to lead to the latter outcome. These findings not only contribute to understanding the effects of CLIL on FL learning, but also shed light on the specific pedagogy-related factors that contribute to FL proficiency development.