This study examined the biliterate writing abilities and development of 25 Spanish-English Latinx children over the course of their first-grade year. As part of their regular classroom instruction, each student created 16 writing compositions over the year, once a month, eight entries each in Spanish and English. Using embedded mixed methods and a holistic view of bilingualism, nearly 400 writing samples were analyzed for various aspects of writing development: ideas expressed, spelling, punctuation use, and grammar, as well as cross-language transfers. Through this analysis, the authors documented the children's biliterate writing growth across time, although growth was not always linear across languages or in all writing abilities. In the beginning of the year, children demonstrated greater abilities in Spanish than English, but as the year progressed, abilities across languages were comparable. This work is significant to the field of bilingual and biliteracy instruction as understandings of biliterate writing development and practices to develop biliteracy are needed.