A number of French-speaking children show difficulties in learning to write, partly as a result of the high complexity of the orthographic system. In order to shed light on the nature of these difficulties, we designed a study which examines the written performances of seven children (mean age 10.0) with learning disabilities (LDS) in comparison to a control group of 22 age-matched normally developing children in a dictation task. Orthographic errors produced by the two groups were analysed according to the linguistic classification of Catach, N., Duprez, D. & Legris, M. (1980, L'Enseignement de l'orthographe, l'alphabet phonétique international, la typologie des fautes, la typologie des exercices. Paris: Fernand Nathan). Analysis revealed: (1) important difficulties with grammatical morphology, both in the control group and in the LDS group, and (2) a predominance of phonetic errors in the productions of the group of LDS children, while these are nearly non-existent in the control group. A follow-up study shows that a second control group of 20 younger normally developing children do not show such a predominance of phonetic errors. Across these three groups, studies revealed that the proportion of phonetic errors increases exponentially with the global number of errors. This finding, together with the LDS children's weakness in phonological awareness, suggests that a restraint at the phonetic level of language, independently of the production modality, may constitute a relevant predictor of upcoming difficulties in the acquisition of written speech.