ABSTRACT How do immigrant parents transmit their origin-country fertility ideals to their children? Drawing on a national sample of immigrants and their children in France (the ‘Trajectories et Origins’ survey), this study argues that the two main aspects of cultural practices in migrant families, religion and language, influence the fertility socialisation of immigrant children in different ways. Religion, often regulating family life and sexuality, directly shapes the ideal family size of immigrant children. Among children of immigrants, those growing up in Muslim families and those in highly religious families desire more children. By contrast, the use of parental origin-country language does not directly affect ideal family size but instead moderates country-of-origin influence. Those who (solely or partly) spoke their parental origin-country language at home during childhood conform more closely to parental origin-country fertility ideals than those who grew up speaking French. These findings highlight that religion and language play different roles in fertility socialisation in migrant families.