A study of code-switching in a group of 35 Spanish-English bilingual third-graders is reported. The students' diary journal entries and writing assignments based on previous classwork are examined. Retelling of stories previously told by the teacher and the journal entries helped identify the kind of language used by students, the code-switching differences in oral versus written language, and the relationship of code-switching to the type or purpose of the writing. The study found evidence of language variation in the form of language interference and code-switching. Samples of interference clearly demonstrate the students' occasional need to fall back on first-language competence to communicate a message. Code-switching instances suggest a difference between previously reported language mixing in speech and code-switching in writing. Written code-switching was limited to single lexical items or very short phrases. In addition, the purpose of the writing appeared to affect code-switching tendencies, with more code-switching appearing in free-writing journal entries than in assigned work. Implications of the findings for the classroom writing environment are discussed. (MSE)