One English speaking child's phonological, semantic, and syntactical development from 15 to 21 months old is detailed in order to investigate the development of the mid vowel categories in English as reflected in spontaneous speech production. Prior research on this topic is summarized to reveal certain trends in the development of mid vowels. This study compared the child's four basic categories of vowels with the full range of an adult's in order to determine correspondences. Realization rules based on these correspondences were postulated. These rules accept adult surface forms as input and give the child's sound categories as output. These are shown to be functionally related. The pattern of mid vowel development from Stage I to Stage II suggests that these vowels can be described by the term regression. It is concluded that phonological acquisition is characterized by an interaction of the inventive nature of the child and the inherent constraints on the phonological category system. (PMJ)