Teacher expectations and judgments about student capabilities are predictive of student achievement, yet such judgments may be influenced by salient dimensions of student identity and invite biases. Moreover, ambitious math teaching may also invite teacher biases due to the emphasis on student-generated inputs and ideas. In this pre-registered audit experiment, we investigate teacher biases in (a) expectations and judgments about student capabilities in math and (b) teacher responsiveness to students' mathematical thinking. Through a between-subjects design, we randomly assigned teachers to a simulated classroom composed of predominantly Black, Latinx/e, or White students and prompted them to respond to a student's mathematical solution. We also prompted teachers to judge the quality of the student's mathematical thinking and rate their expectations about the difficulty of the problem for the typical student. Our findings show teachers expected greater task difficulty in both the Latinx/e and Black classroom conditions relative to the White. We also found teachers may be more likely to support student sense-making and provide more positive, substantive affirmations to Black students relative to White students for the same mathematical solution. We did not find differences by condition in other dimensions. Our findings have implications for teacher training and reform-oriented mathematics instruction.