The major purpose of this paper is to initiate discussion on the validity of systematic phonemics in the area of language acquisition. This is not an attempt to write a phonology, but rather an outline of some theoretical and formal devices that may be used for gaining insight into the phonological system of the child. An evaluation procedure suggested is that of "developmental adequacy" which accounts for the process by which a child moves closer to a system that is constantly presented to him. Such a procedure includes statements of stability that mark certain aspects of the child's system as similar to, and others as distinct from, the model to which he is presented. A summary of rules suggested for the child's system include: reduplication, diminution, production alternation, and adaptation. The productive rule is introduced to cover the child's comprehension of sounds he does not yet produce, and the adaptive rule expands the notion of "developmental adequacy." (Author/LG)