Two experiments used a mutual imitation paradigm to assess 9-, 14-, and 18-month-old infants' developing understanding of intentions in others. In the first study, 1 experimenter imitated the infants' actions, and another experimenter performed contingent but different actions on an identical toy. From 9 months of age, infants show discrimination between the mimicking and the contingent experimenter. In a second study, same-age groups of infants faced either an experimenter mimicking their actions on an identical object or the object mimicking them independently of any manual contact by the experimenter. Only 14- and 18-month-olds showed discrimination between the 2 conditions, this discrimination correlating with infants' relative ability to follow gaze and points in triadic exchanges. These results are interpreted as demonstrating important developmental changes between 9 and 14 months in the construal of others as intentional. (Contains 4 tables and 9 figures.)