The present research examined whether 5- to 6.5-month-old infants would hold different expectations about various physical events involving a box after receiving evidence that it was either inert or self-propelled. Infants were surprised if the inert but not the self-propelled box: reversed direction spontaneously (Experiment 1); remained stationary when hit or pulled (Experiments 3 and 3A); remained stable when released in midair or with inadequate support from a platform (Experiment 4); or disappeared when briefly hidden by one of two adjacent screens (the second screen provided the self-propelled box with an alternative hiding place; Experiment 5). On the other hand, infants were surprised if the inert or the self-propelled box appeared to pass through an obstacle (Experiment 2) or disappeared when briefly hidden by a single screen (Experiment 5). The present results indicate that infants as young as 5 months of age distinguish between inert and self-propelled objects and hold different expectations for physical events involving these objects, even when incidental differences between the objects are controlled. These findings are consistent with the proposal by Gelman, R. (1990). First principles organize attention to and learning about relevant data: Number and the animate-inanimate distinction as examples. "Cognitive Science, 14", 79-106, Leslie, A. M. (1994). ToMM, ToBY, and Agency: Core architecture and domain specificity. In L. A. Hirschfeld & S. A. Gelman (Eds.), "Mapping the mind: Domain specificity in cognition and culture" (pp. 119-148). New York: Cambridge University Press, and others that infants endow self-propelled objects with an internal source of energy. Possible links between infants' concepts of self-propelled object, agent, and animal are also discussed.