Empirical analyses of parimutuel betting markets have documented that market probabilities of favorites (longshots) tend to underestimate (overestimate) the corresponding empirical probabilities. We argue that this favorite-longshot bias is consistent with bettors taking simultaneous positions on the basis of private information about the likelihood of different outcomes. The ex post realization of a high market probability indicates favorable information about the occurrence of an outcome -- and the opposite is true for longshots. This explanation for the bias relies on the bettors' inability to incorporate the surprise revealed by the final odds.