This article describes a case study based on data from 2 surveys and 2 retrospective interviews with 2 preservice teachers and 1 interview with their 2 mentors. All 4 teachers worked in the same classroom in a field-based, postbaccalaureate, urban teacher certification program. 1 observation was also conducted in the classroom of each preservice teacher after she began teaching. Although the preservice teachers worked with mentors who modeled standards-based writing instruction, movement toward such teaching did not occur when preservice teachers' beliefs about learning and teaching were inconsistent with mentors' beliefs. Conceptual development of 1 preservice teacher was a process of gradual socialization, whereas the other experienced conceptual conflict. Preservice teachers' initial beliefs, mentors' teaching and mentoring practices, and school contexts influenced preservice teachers' conceptual development.