When Mayotte Island became an overseas “department” in 2011, the local Muslim civil law was transformed so as to be similar to French civil law. This article examines the sociological and juridical effects of this change. The Muslim judges no longer have any legal functions. They only register Muslim marriages that are consistent with moral and social norms but not recognized by the law. The new civil law applied in Mayotte changes definitions of kinship and paternity ties (marriage, filiation) and contributes to new gender relations. The migration of Comorians from neighboring Comoros islands also has an effect on gender relations and on the situation of children separated from their parents.