Williams syndrome (WS) is a genetic syndrome of abnormal neurodevelopment, characterised by a specific linguistic pattern. Comparing performances of WS subjects with those of normal children in a word span task, we found that WS subjects revealed normal phonological similarity and length effects but a reduced frequency effect. Our results suggest comparable phonological encoding mechanisms in WS and normal controls and, at the same time, it provides evidence for an impaired access to lexical-semantic knowledge in WS subjects. This dissociation fits well with the particular pattern of linguistic abilities of these subjects.