Children acquire the meaning of ook ‘also’ in Dutch relatively late ( Bergsma 2006 ), although this focus particle is highly frequent. We argue that this late acquisition is caused by a pragmatic rule: contrastive implicature. We follow Sæbø (2004) , who argues that additives are used because without them, the sentences they appear in would be interpreted as contrastive in relation to the context. Data from a sentence completion task administered to Dutch L1 learners (N = 62, ages 4;0–5;11) show that, on average, four-year-olds do not distinguish sentences with ook from sentences without ook. Five-year-olds do better on sentences with ook but worse on sentences without it. We argue that they have generally acquired contrastive implicature: they apply the correct contrastive interpretation to sentences without ook, but overgeneralize this implicature to sentences with ook, before completely acquiring the meaning of ook.