Students' beliefs that mathematical abilities are fixed can cause long-standing problems with motivation and learning. Hence, teachers should notice such problematic beliefs about identity among their students and handle them adequately. We used written descriptions of classroom situations to determine whether preservice mathematics teachers (n = 80) noticed fixed beliefs about mathematical abilities and whether they had strategies for dealing with them. The qualitative data were coded and transformed into a score for noticing. We found that most of the preservice teachers did not notice problematic beliefs. Thereby, preservice teachers who believed that mathematical abilities are malleable were more likely to notice fixed beliefs among students. When describing beliefs, few participants referred to theoretical concepts. Hardly any preservice teachers suggested strategies for handling students' beliefs. However, the strategies that were suggested mostly corresponded with findings from educational research. Our study provides first evidence that preservice teachers' abilities to notice and handle belief-related problems may be insufficient. We discuss implications for teacher education as well as directions for future research.