This paper describes a case study of 2 departments in a school, where teachers were inquired about their school's needs for improvement and how the institution and their department impacted upon their own professional development. The results show that the teachers' experiences of professional development, their interpersonal relationships within their department, and their views about school improvement differed radically between the 2 organisational subunits. Results are discussed on the basis of their implications for whole-school improvement efforts in departmentalised, loosely coupled educational institutions. The paper makes a case for the continuing relevance of models of schools as loosely coupled organisations and for the combination of social network analysis and qualitative research methods to illustrate how loose coupling manifests itself in schools and how it impacts on teacher development and on school capacity for improvement. (Contains 6 tables, 6 sociograms and 6 notes.)