Background/Context: Kindergarten mathematics instruction is critical for students' future academic success. The nature and quality of this instruction may vary depending on classroom characteristics. However, little empirical work has examined how mathematics instruction in kindergarten might differ based on classroom performance levels. Purpose, Objective, Research Question, or Focus of Study: This study focuses on whether kindergarten teachers' mathematics instructional practices differ based on reported performance levels of students in the classroom. In particular, we focused on whether mathematics instructional time, as well as the extent to which teachers use traditional versus ambitious mathematics practices, differed based on teaching a higher proportion of children performing below grade level. Research Design: This study used a nationally representative dataset of approximately 2,900 kindergarten teachers from the Early Childhood Longitudinal Study, Kindergarten Class of 2010-11. With these data, the study utilized a model that compared teachers to each other within the same school (i.e., school fixed effects). Conclusions or Recommendations: This study found that mathematics instructional time did not differ based on the proportion of children reported to be below grade level in the classroom. However, how teachers taught mathematics differed: teachers who reported having a greater percentage of students performing below grade level used ambitious practices less frequently. These findings were not moderated by measures of teacher background or teachers' reported expectations of their students.