Aiming at internationalisation, the Japanese government initiated the Japan Exchange and Teaching (JET) Programme in 1987 by introducing team-teaching by Japanese teachers of English (JTEs) and young graduates from overseas as Assistant Language Teachers (ALTs) into English language classrooms throughout Japanese public schools. Previous studies have shown that there have been, in some cases, a lack of interaction and collaboration between the JTEs and ALTs. However, these studies tended to focus mainly on classroom activities and consequently the teams were found to be the main cause of ineffective teaching and learning. In order to gain a deeper understanding of these team-teaching interactions and problems, the conceptual framework of this ethnographic case study was based on Holliday’s concept of small cultures and Lave and Wenger’s theory of communities of practice. It aimed to explore how team teachers’ (JTEs’ and ALTs’) perceived and actual roles are influenced by school cultures, as well as which aspects of the school cultures could impact on the effectiveness of the JET Programme. It also investigated the ways in which the ALTs are welcomed and accepted into the schools as well as how the ALTs cope with the contexts of specific school settings and cultures. Data for the study were obtained from 4 ALTs, 7 JTEs, 3 administrative teachers and 1 PE teacher through semi-structured individual interviews, observations of team-taught lessons of 5 teams and school activities outside the classrooms which ALTs attended, as well as fieldnotes, artefacts and documents. The findings revealed the complexities of the school cultures (as small cultures) and the effect that these cultures have on the perspectives of ALTs and JTEs, their roles and contributions and on the relationships between the ALTs and JTEs. The cultures also strongly affected the roles of schools as teachers’ professional learning and development communities (communities of practice). This study suggests that the JET Programme needs to be tailored to the specific culture, and that influential people in each school should be involved to overcome any difficulties caused by cultural aspects. These approaches may create supportive professional development communities within the schools and improve collaboration between the JTEs and ALTs.