The question is addressed whether a child would develop a communication system if a conventional linguistic model is absent. Six congenitally deaf children, who were not exposed to Sign Language, were observed and videotaped at play in their homes at intervals of one to three months. The children ranged in age from 1 year, 5 months to 4 years, 1 month at the first interview. The study demonstrated that, like natural languages, the deaf child's system can best be described by positing an underlying structure for both simple and complex sign phrases. The surface measure investigated was production probability, that is, that an element will be explicitly signed when that element can be signed. It was found that production probabilities were not constant across different types of actors and patients in the children's simple phrases. Therefore, a second descriptive level was posited, called underlying structure, as part of the deaf child's communication system. Both actor and patient surface measures provided evidence for underlying structure in both simple and complex phrases. One conclusion from this research is that a young child can develop this underlying structure with little help from the linguistic environment. (AMH)