The authors examined inflectional and derivational morphological forms within narratives written by 247 3rd and 4th graders. Results indicate that children's control of morphological structures in their writing mirrors that in their speech: inflectional morphology is largely mastered by age 9 or 10, but skills with derivational morphology continue to develop in middle childhood. (Contains 49 references, 5 tables, and 1 appendix.) (GCP)