In a computerized task, an adult with intellectual disabilities learned to construct consonant-vowel-consonant words in the presence of corresponding spoken words. During the initial assessment, the participant demonstrated high accuracy on one word group (containing the vowel-consonant units "it" and "un") but low accuracy on the other group (containing the units "ag" and "ed"). Errors occurred almost exclusively in the vowel position. Training the group with low accuracy increased accuracy on the trained word group but decreased the accuracy on the untrained, but initially accurate, group. High accuracy on both groups together occurred only after added training in which all five vowels ("a," "e," "i," "u," and "o") were taught together in words that differed only in the vowel. These findings indicate the importance of carefully arranging examples and nonexamples to sharpen stimulus control. The findings also illustrate a promising step in the development of effective instructional programming for remediation of medial-vowel errors, which can be a source of difficulty in early reading instruction. (Contains 1 table and 3 figures.)