From birth on, children are confronted with an ever-growing variation of languages. In Dutch primary schools the main language model nowadays is a submersion model centring round Standard-Dutch. The disappointing results of several small-scale bilingual experiments and the large-scale implementation of a bilingual model for immigrant children in the past had led the Ministry of Education to abandon bilingual education, that is, with the exception of models with English, German or French as a second language. The focus of the present article is on language variety at home and school. The main question is how the mother tongue, viz. the language the parents speak, influences their child’s proficiency in Dutch. Analysing data of 14,000 grade 2 pupils that were collected in the 2007, 2010 and 2013 measurement waves of the national COOL5–18 study, this paper first of all describes how often children speak their mother tongue. A distinction is made between Dutch; Frisian and Dutch regional languages and dialects; and foreign languages. The latter mostly refers to Turkish and Berber language varieties. Next, the correlation between speaking the mother tongue and the children’s proficiency in Dutch is analysed. The results show that for children of Dutch, Frisian and Dutch regional languages and dialect speaking parents there are no differences in level of Dutch relative to the number of times they speak their mother tongue. For children of a foreign language speaking parents, however, there is a tendency that the more often they speak their mother tongue the lower their proficiency in Dutch is. This negative relationship is not reduced by accounting for the mother’s educational level and proficiency of Dutch. Most of the children who speak a foreign language are of Turkish or Moroccan descent who in terms of school achievement lag considerably behind their native-Dutch peers. It is suggested that implementing so-called transition classes with targeted language courses during one school year and a follow-up in later years may help diminish this gap.