This study examines the writing skills of students in grades 1 to 5 of an Italian-English two-way immersion program. Narrative writing samples were collected in both languages and scored for content, organization, grammar, mechanics, vocabulary, text length, and overall total score. The results show that writing skills improved as a function of grade. Although children from Italian-English bilingual homes obtained higher Italian scores than English-only students, both groups achieved moderate-to-advanced levels of writing proficiency in both languages by the upper grades, suggesting that the program was successful in promoting bilingualism and biliteracy. Total scores and scores for the various subcomponents were positively and significantly correlated across languages, even after controlling for grade. Associations were stronger for global components like content and organization; smaller but significant correlations were also found for language-specific measures like grammar and vocabulary. We interpret these findings as evidence of the mutual influence of L1 and L2 writing skills in typologically and orthographically related languages.