Pupillary contagion—responding to pupil size observed in other people with changes in one’s own pupil—has been found in adults and suggests that arousal and other internal states could be transferred across individuals using a subtle physiological cue. Examining this phenomenon developmentally gives insight into its origins and underlying mechanisms, such as whether it is an automatic adaptation already present in infancy. In the current study, 6- and 9-month-olds viewed schematic depictions of eyes with smaller and larger pupils—pairs of concentric circles with smaller and larger black centers—while their own pupil sizes were recorded. Control stimuli were comparable squares. For both age groups, infants’ pupil size was greater when they viewed large-center circles than when they viewed small-center circles, and no differences were found for large-center compared with small-center squares. The findings suggest that infants are sensitive and responsive to subtle cues to other people’s internal states, a mechanism that would be beneficial for early social development.