The present study examines whether literacy or phonological impairment affects use of morphological spelling constancy; the principle that morphemes are spelled consistently across words. Children with dyslexia or otitis media (OM) were compared to chronological-age matched children and reading-ability matched children. Monomorphemic and polymorphemic nonwords were spelled in a sentence completion dictation task. Use of root and suffix morphemes increased with age in typical development, particularly derivational morphemes. Dyslexic children generally used morphological strategies less than their chronological-age matched peers but to a similar extent as reading-ability matched. OM children showed a specific weakness in using inflectional suffixes. Results suggest different causes for the spelling difficulties in each case: dyslexic children had difficulties in generalising more complex morphological relationships, while the OM children’s difficulties had a phonological/perceptual basis.