This research examined the development of adaptive generalization in infants' object-directed actions. Infants ages 9 and 12 months participated in an object manipulation task with stimulus objects from 2 categories that differed in shape and weight and that bore a consistent shape or weight correspondence. Weight differences between categories affected infants' actions required to handle objects effectively. Infants manually explored objects from both categories and then were tested for their selection of different actions between categories and their generalization to novel exemplars within categories. Nine-month-olds provided no evidence of category differentiation and generalization; however, 12-month-olds adapted their actions selectively for objects of each category and generalized those actions to novel objects within categories. A second sample of 9-month-olds who were examined in a simplified task with just one object per weight level successfully adapted their actions by weight. Together, the findings provide evidence for the development of selection and generalization in manipulative action across the second half of the first year of life.