Educational material often includes engaging perceptual information. However, this perceptual information is often extraneous and may compete with the deeper to-be-learned structure, consequently hindering either the learning of relevant structure or its transfer to new situations. This hypothesis was tested in 4 experiments in which 6- to 8-year-old children learned to read simple bar graphs. In some conditions, the bars were monochromatic (i.e., No Extraneous Information), whereas in other conditions, the bars consisted of columns of discrete countable objects (i.e., Extraneous Information). Results demonstrated that the presence of extraneous information substantially attenuated learning; participants tended to count the objects and failed to acquire the appropriate strategy. The interference effects decreased with age. These findings present evidence of how extraneous information affects learning of new mathematical knowledge. Broader implications of these findings for understanding the development of the ability to filter task-irrelevant information and for educational practice are also discussed. (Contains 7 figures.)