Data are presented from one subject (J) that show a gradual development of the complexity of words in terms of syllable structure and degree of phonetic similarity of co-occurring consonants. During the age range of 1;9 to 2;6, J's data show a highly systematic progression of stages, each characterized by fewer restrictions on the number, order, and type of syllables and on the co-occurrence of consonants than earlier stages. The possibility is noted that the systematicity of the corpus may be related to the fact that J was a non-imitator for the first four months of testing. Several types of data support the interpretation that the limits on complexity seen at each stage of development are limits on the child's production (not perception or storage) at that time. It is suggested that, for this child, the "word" is the unit of production performance in terms of which the phonological complexity is constrained. Within such a context, the limitations on complexity seen in J's phonological data may reflect a general psychological process, similar to that noted in other aspects of language acquisition. (Author/NCR)