Preschoolers use outcomes of actions to infer causal properties of objects. We asked whether they also use them to infer others' causal abilities and knowledge. In Experiment 1, preschoolers saw 2 informants, 2 tools, and 2 broken toys. One informant (the "labeler") knew the names of the tools, but his actions failed to activate the toys. The other (the "fixer") was ignorant about the names of the tools, but his actions succeeded in activating the toys. Four-year-olds (and to a lesser extent, 3-year-olds) selectively directed requests for new labels to the labeler and directed requests to fix new broken toys to the fixer. In a second experiment, 4-year-olds also endorsed a fixer's (over a nonfixer's) causal explanations for mechanical failures. They did not, however, ask the fixer about new words (Experiments 1 and 2) or artifact functions (Experiment 1). Thus, preschoolers take demonstrated causal ability as a sign of specialized causal knowledge, which suggests a basis for developing ideas about causal expertise. (Contains 1 figure.)