The purpose of this study is to describe and analyse the religious assimilation of the 17th century Walloon minority in the mining district of Dannemora in the Swedish province of Uppland. After a general survey of religious conditions in the homeland of the Walloons and in Sweden, the Walloons' relation to the Church of Sweden is studied against the background of the more and more rigorous legislation. Their religious assimilation is analysed by investigating Calvinistic matters mentioned in the minutes and acts of the Cathedral chapter of Uppsala and marriages, baptisms and funerals recorded in the parish registers. Even if their religious assimilation was not total at the end of the 17th century, most Walloons belonged to the Swedish Church. Due to continued religious instruction, however, some of the 460 adults of the Walloons still adhered to Calvinism. Especially noted as Calvinists are the families Baudou and Dandenel.