The linguistic interactions in the speech of bilingual children is systematically examined to determine their ability to differentiate between their two language systems. The speech samples of eighteen bilingual (Spanish-English) children aged two to six were examined for instances of deviation from the norms of the language of utterance due to familiarity with more than one language. Language mixing within a sentence boundary ("Yo tengo un car,""I have a car") and phrasal interaction ("I ask him que yo voy a casa," I asked him if I should go home) are considered. Language switching at the sentence boundary (e.g., Experimenter: "What are you doing?" Child: Estoy jugando con mis juguetes") is also discussed. Tabulated statistics reveal the breakdown of the linguistic interactions. Only 1.7% of the children's speech contains language mixing, usually the insertion of an English noun into a Spanish environment. That the children are conscious of the mixing is evident from its consistency. Language switching accounted for only 1% of the children's speech. Children appear to mix and switch for three reasons: (1) to test or mock the experimenter, (2) because the child momentarily forgets the correct word, or (3) the mixed word was more salient in the child's environment. (PMJ)