Purpose: Teachers' ability attributions play an important role in students' educational outcomes. Perceptions of academic abilities, however, are subject to biases. This study aims to examine ethnic biases in homeroom teachers' ability attributions in Hungarian primary schools. Design/methodology/approach: Using a unique database combining survey data collected among teachers with administrative data on students' standardised test scores, the author compares ability attributions towards equally competent minority and majority classmates (Nstudents = 604, Nclasses = 34 in Grade 6; Nstudents = 420, Nclasses = 27 in Grade 8). Findings: The author finds that Roma students are less likely to be perceived as smart by their homeroom teachers than their non-Roma classmates with similar standardised achievement scores in Grade 6, but not in Grade 8. The ethnic difference in being perceived as smart is substantially reduced after controlling for students' socioeconomic status and cultural resources. On the other hand, homeroom teachers perceive Roma students to be similarly hardworking and "good students" than equally competent non-Roma students. Originality/value: This study highlights an important mechanism that can contribute to educational inequalities. The findings suggest that previously found differences between equally competent Roma and non-Roma students' teacher-given school grades might arise due to biases in ability attributions rather than differences in perceived efforts. It is important to make teachers aware of potential biases in student assessment and evaluation.