Mongoloid children appear to be useful candidates for studying difficult methodological problems found in developmental research. They represent a subpopulation in which general development is markedly slower than in the normal child and in which language development appears to be more dependent on age than general intellectual development. This report discusses some important considerations for studying a child with a restricted linguistic system, including data collecting, segmentation and glossing of utterances, and speech style. The subject of this study was a five-and-a-half-year-old mongoloid boy who was studied over a two-day period, and from whom 270 utterances were collected. Forty percent of the child's utterances were intelligible and could be segmented by intonational contours, pauses, change of speakers, and mean length of utterance. Establishing new criteria for glossing unintelligible utterances enabled most of the rest of the corpus to be glossed. An analysis of the glossable utterances classified as conversation showed that the child's style was characterized by: (1) volume changes, (2) phonetic changes relating to the other person's speech, (3) rephrasing when misunderstood, (4) vocal and gestural annoyance, and (5) gestures complementing the conversational context. A summary of the phonological analysis is provided. (Author/LG)