Research suggests discipline-specific, educative mentoring can help preservice teachers develop more sophisticated pedagogical content knowledge (PCK). However, there are few studies examining the nature of mentors' practice and "how" mentors influence preservice teacher's (PST) PCK. The purpose of this case study was to describe the strategies used by a secondary biology mentor teacher to support the development of a PST's PCK. The primary data sources were the transcripts of audio-recorded, daily meetings between the mentor and the PST during two curriculum units: DNA/Protein Synthesis and Evolution. The mentor influenced the PST's teaching orientation by repeatedly comparing teacher- and student-centered approaches, asking him to consider how students learn, and asking him to self-assess whether his instruction aligned with his teaching beliefs. The mentor helped the PST develop topic-specific knowledge of instructional strategies by sharing strategies she used previously, modeling critical reflection, and inviting him to critically reflect on his own instructional strategies. Topic-specific knowledge of students' understanding of science was developed by discussing common student misconceptions revealed in students' conversations and by sharing the results of test-item analysis from previous unit tests. The mentor helped develop the PST's topic-specific knowledge of assessment by helping him critically analyze and revise previous examinations to better align with the current curriculum units. Topic-specific knowledge of curricula was developed by jointly grappling with decisions about concept sequencing within units. The study includes implications for research, science teacher education, and professional development for mentors.