Whereas most research on child second language acquisition has focused on the oral production of children, studies are needed to illuminate patterns of literacy development in a second language. This paper aims to shed some light on this process, focusing on the English writing development of a native Spanish speaker, and on the Spanish writing development of a native English speaker, both enrolled in a two-way immersion program in the USA. Data for this particular study come from a longitudinal, national study, involving 484 students in 11 programs across the country. In this paper, we will present a case study of two students in the project, from the beginning of third grade through the end of fifth grade. The students' writing samples will be discussed using an analytic rubric developed for the project. This rubric addresses three major aspects of writing--composition, grammar and mechanics--and provides a clear framework for investigating changes in writing ability over time.