Research with adults has shown that the distortion of visual word features, and in particular of the multiletter features within words, hampers word recognition. In this study, "CaSe MiXiNg" was employed to examine the effect of disrupting visual word features on the acquisition of orthographic knowledge in children. During the training, 18 beginning and 27 advanced readers (in Grades 2, 4, and 5) repeatedly read a set of pseudowords in either lowercase or mixed case. During this training, case mixing appeared to impair reading speed in both reader groups. At posttest, 1 day after the training, case format was either the same as or different from that during the training. Lowercase pseudowords were recognized faster after a lowercase training than after a mixed-case training. In a second study, case was found not to affect the rapid naming of single letters. The combined results suggest that case mixing disrupted the multiletter features in pseudowords and that the disruption of these features can affect the acquisition of orthographic knowledge.