Background. Written composition requires handwriting, spelling, and text planning skills, all largely learned through school instruction. Students' rate of learning to compose text in their first months at school will depend, in part, on their literacy-related abilities at school start. These effects have not previously been explored. Aim. We aimed to establish the effects of various literacy-related abilities on the learning trajectory of first-grade students as they are taught to write. Sample. 179 Spanish first-grade students (94 female, mean age 6.1 years) writing 3,512 texts. Method. Students were assessed at start of school for spelling, transcription fluency, letter knowledge, phonological awareness, handwriting accuracy, word reading, and nonverbal reasoning. They were then taught under a curriculum that included researcherdesigned instruction in handwriting, spelling, and ideation. Students' composition performance was probed at very regular intervals over their first 13 weeks at school. Results. Controlling for age, overall performance was predicted by spelling, transcription fluency, handwriting accuracy, word reading, and non-verbal reasoning. Most students showed rapid initial improvement, but then much slower learning. Weak spellers (and to a lesser extent less fluent hand-writers) showed weaker initial performance, but then steady improvement across the study period. Conclusion. Transcription ability at school entry affects response to writing instruction. Most children start school with a well-developed ability to compose their thoughts in speech, but without the ability to compose their thoughts in writing. Most obviously, this is because the ability to spell and handwrite requires explicit instruction, and in most educational systems, this instruction does not start in earnest until the beginning of first grade. Written composition also requires a different approach to retrieving ideas. A parent or teacher asking a child to speak a story can interject with 'Where were they?', 'What This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.