Background: Traditionally writing instruction at the start of school has focused on developing students' ability to spell and handwrite. Teaching children explicit self-regulatory strategies for developing content and structure for their text has proved effective for students in later grades of primary (elementary) education. Aims: The present study aims to determine whether first-grade students benefit from learning higher-level self-regulating strategies for explicit planning of content and structure. Sample: Five mixed-ability Spanish first-grade classes were randomly assigned either to an experimental condition that received strategy-focused instruction (three classes, N = 62), or to a practice-matched control condition (two classes, N = 39). Method: Over 10, 50-min sessions, the intervention taught strategies for writing stories. Writing performance was assessed prior to intervention, immediately after intervention and 7 weeks post-intervention, in terms of both text features associated with written narratives and by holistic quality ratings. Results: Students who received the intervention subsequently produced texts with better structure, coherence, and quality, and a larger number of features associated with narrative texts. These effects remained at follow-up and were not present in the control condition. Conclusion: Our findings indicate that teaching explicit strategies for planning text content and structure benefits young writers even when spelling and handwriting skills are not yet well established.