The influence of teacher beliefs on curriculum implementation was explored in two different contexts. Two middle-level teachers implemented a 20-lesson curriculum package in the presence of participant observers. Data sources included field notes and interview transcripts. The data were reviewed daily and tentative assertions were developed. Assertions and their accompanying evidence were examined weekly for recurring patterns. Case studies of the belief structures of each teacher were then prepared. In both cases, four major categories of beliefs appeared to influence curriculum implementation. These included beliefs about how students learn, a teacher's role in the classroom, the ability levels of students in a particular age group, and the relative importance of content topics. Both teachers believed that the most important student outcome is factual knowledge, that middle-grade students learn through repeated drill and practice, and that middle school students require a great deal of direction. The teachers' beliefs differed in other areas, including beliefs about a teacher's role in the classroom and beliefs regarding the curriculum and its content topics. Although certain components of both teachers' belief structures enhanced the success of curriculum implementation, overall their existing belief structures were incongruent with the underlying philosophy of the intended curriculum, thus hampering successful implementation.