The students in 21st-century public middle schools are increasingly diverse in terms of language proficiency, cultural and ethnic representation, and varied levels of poverty; and, yet, they are being educated in a political climate that encourages mainstreaming special education and gifted services in the regular classroom. Given this context, this study sought to examine 48 middle school content-area teachers' beliefs about teaching in diverse classrooms to determine how these beliefs affected their willingness and capacity to differentiate their instruction and assessment. A qualitative study design incorporating grounded theory methodology (Glaser & Strauss, 1967; Strauss & Corbin, 1990) was employed. Four teacher beliefs emerged from interview, observation, and document data that conflict with the philosophy undergirding differentiation. Each belief is presented with supporting evidence from the data and discussed in terms of its relationship to effective differentiated classroom practices. (Contains 1 table.)