This research investigated different forms of sex bias in teacher expectations relative to gymnastics performance. First, a laboratory experiment including 163 physical education teachers confirmed that stereotypes favorable to boys may influence teacher expectations in gymnastics. Next, a naturalistic study involving 15 teachers and 422 students showed that teachers expected no sex differences even though girls performed higher than boys. However, this sex bias was due to the reliance on non-diagnostic student personal characteristics favorable to boys rather than on a stereotype per se. These results suggest that egalitarian beliefs may mask a bias in favor of a social group when group differences do really exist, and that sex-biased teacher expectations do not inevitably involve an influence of sex stereotypes.