This study investigates the speech of a four year old girl who is growing up in a Chicano neighborhood in Houston. Her parents, and many of her neighbors, mix English and Spanish. The girl's linguistic system is described and modeled from a cognitive perspective. The girl was observed interacting with family and friends at her home. Her styles of speech in different social situations were analyzed. Different linguistic subsystems are apparent at different levels. Her phonology is one composite system. She has separate English-like and Spanish-like morphological constructions for verbs and pronouns, but subsystems of morphological constructions for nouns are fuzzier. Syntactically, English-like and Spanish-like subsystems are clearly revealed for verb phrases, fuzzily revealed for clauses, whereas noun phrases are not organized into subsystems. Four subsystems of lexical classes can be discerned based on their use in different interpersonal roles. She has different ways of speaking to babies, younger playmates, parents, and older friends. The girl's speech is generally appropriate for each situation. However, the variation is not due to a consistent distinction between English and Spanish. Code-switching models do not reflect the organization of her linguistic system. Rather, she organizes her linguistic knowledge as a collection of signs, and various relations between signs account for her appropriate linguistic behavior in a given situation.