Literacy acquisition in children’s L1 and L2 contributes to academic success, and embraces and supports children’s backgrounds and identity formation. However, limited education can prevent bilingual children from developing their writing fluency on the same level in L1 and L2. An analysis of writing fluency can show aspects that require more or less effort in children’s L1 and L2 writing. Thus, it is important to examine writing fluency and the related reading, linguistic and cognitive skills across children’s languages. Our knowledge is limited regarding the skills that could influence children’s writing fluency. Previous studies have mainly focused on Latin scripts and/or one alphabetic and one non-alphabetic script. Furthermore, English has been the writers’ L1 or L2. This paper investigated reading, linguistic and cognitive skills related to the writing fluency of bilingual biscriptal children in two different alphabetic scripts: Persian (L1) and Swedish (L2) across two different genres: narrative and descriptive. 23 children in years 4–9 (aged 10–15) produced four texts each using the Eye and Pen tool. Standardised tests across both languages were used to explore the participants’ reading, linguistic and cognitive skills. Analyses showed that they were more fluent writers across both genres in L2. Word reading appears to contribute to writing fluency across both languages, whereas vocabulary knowledge only related to writing fluency in L1. No significant relationship was found between working memory and writing fluency in either L1 or L2 © 2023 International Association for Research in L1-Education