This qualitative study investigated the progression in the biliterate levels of conceptualization about writing in Spanish and English of 28 simultaneous bilingual preschoolers. Using a constructivist framework, a holistic bilingual and translanguaging perspective, this qualitative study examined the children's understanding of the nature of writing in Spanish and English as demonstrated in their written productions and interpretation of their own texts as well as how such understanding develops over time. The findings indicate that while similar conceptualization levels to those found in previous studies with monolingual children were identified in the simultaneous bilingual children in our sample, some unique patterns were observed that shed light on the nature of early bilingual writing development. More importantly, the findings reveal that, from the start, most children's progression in their levels of conceptualization about writing in the two languages was similar, even when they did not receive formal literacy instruction in English. (author abstract)