Stigmatization consequent to anti-fat bias (AFB) may affect the services people who are obese receive from health professionals, including physical education and exercise science (PEX) professionals. In this study, we compared AFB levels of American and Mexican PEX students and Mexican athletes. We also investigated if socially desirable (SD) response tendencies threaten the validity of the explicit AFB measure used in this study. Participants (N = 118) completed measures of explicit and implicit AFB. Explicit AFB scores were not different between groups, but there were some subsample differences and interactions on the implicit AFB measure. Most implicit AFB subsample scores were significantly different from 0, indicating the participants were, to some extent, implicitly biased against fat people. The correlations between SD scales and the explicit AFB scales indicated no substantive threat to the validity of those scales. These results indicate AFB may be an issue with future PEX professionals, and thus, further research on incidence levels and prevention strategies is warranted. Also, because explicit and implicit AFB scores were not significantly correlated, researchers should investigate whether implicit bias affects an individual's explicit bias.