Overview: This research explores using a teaching approach that attempts to balance test preparation with creating "teachable moments" for students. This approach involves the use of a sequence of assessments to introduce topics through formative assessment in order to identify students' understanding, and beginning instruction based on an evaluation of students' knowledge and alternative conceptions. In an attempt to balance classroom instruction and large-scale test preparation, this science teacher attempted to use a teachable moment assessment approach as a teaching tool. Setting: The study was conducted at a Northern California high school whose student population (N = 350) was 55% African American, 40% Hispanic, 2% Pacific Islander, 1% Asian, 1% Filipino and White, and 1% nonreporting. The ethnographic study was conducted with 1 teacher working with a classroom of 25 students in their first high school biology class. Method: Over the course of 6 weeks of instruction, we documented the teacher's use of teachable moment assessments to promote students' use of discourse in relation to the teacher's use of this approach to teaching. We videotaped each session over the course of 6 weeks and collected their classroom journals in an effort to conduct post hoc domain analysis of students' discourse practices. Findings: The findings indicate that using assessments as a pedagogical tool provided the teacher with an opportunity to isolate students' learning in three areas: students' (1) conceptual understanding, (2) analysis skills, and (3) use of science discourse. These results were consistent with the theoretical intent of developing students' identity across conceptual, analytical, and discursive lines. These findings are indicative of the fact that sound classroom instruction and preparation for testing are not mutually exclusive. Future research must explore ways to promote synergistic relationships between teachable moment instruction and test preparation.