This paper deals with the assessment of children's acquisition of phonological competence. Phonological competence refers to the idealized speaker/hearer's ability to understand the linguistic structure underlying phonetic input/output. Under the Jakobsonian assumption that the learning of phonological opposition follows a regular sequence, an index of phonological competence in the form of a speech sound discrimination test is proposed. Developmental articulation data are used to construct a model of emerging phoneme discrimination proficiency. The model is intended as a basis for the development of practical assessment procedures. (Author/RB)