This qualitative longitudinal study examines the impact of an intensive eight-week in-service teacher education programme in the UK on the beliefs of six English language teachers. Drawing on a substantial database of semi-structured interviews, coursework and tutor feedback, the study suggests that the programme had a considerable, if variable, impact on the teachers' beliefs. The course allowed teachers to think more explicitly about, become aware of, and articulate their beliefs, to extend and consolidate beliefs they were initially--and sometimes tacitly--positively disposed to, and to focus on ways of developing classroom practices which reflected their beliefs. Teachers also experienced shifts in prior beliefs they held about aspects of language teaching and learning. Nonetheless, despite this evidence of impact, the data also suggest that the in-service course studied here could have engaged teachers in a more productive and sustained examination of their beliefs. Several factors relevant to such engagement are analyzed and recommendations for enhancing the impact of in-service teacher education on language teachers' beliefs are made. (Contains 2 tables.)