In 2 studies, the sensitivity of 3- and 4-year-olds to the previous accuracy of informants was assessed. Children viewed films in which 2 informants labeled familiar objects with differential accuracy (across the 2 experiments, children were exposed to the following rates of accuracy by the more and less accurate informants, respectively: 100% vs. 0%, 100% vs. 25%, 75% vs. 0%, and 75% vs. 25%). Next, children watched films in which the same 2 informants provided conflicting novel labels for unfamiliar objects. Children were asked to indicate which of the 2 labels was associated with each object. Three-year-olds trusted the more accurate informant only in conditions in which 1 of the 2 informants had been 100% accurate, whereas 4-year-olds trusted the more accurate informant in all conditions tested. These results suggest that 3-year-olds mistrust informants who make a single error, whereas 4-year-olds track the relative frequency of errors when deciding whom to trust.