[Introduction] In the controversies over immigration and Islam in the early 21st century, Muslims have widely become associated with religious fundamentalism. Others have argued that religiously fundamentalist attitudes characterize only a small minority of Muslims living in the West, and can be found to similar extents among adherents of other religions, including Christianity. Claims on both sides of this debate lack a sound empirical base. Little is known about the extent and determinants of religious fundamentalism among Muslims of immigrant origin, and virtually no evidence is available that allows a comparison with Christians of native stock. Whether religious fundamentalism among Muslims should be considered as a relatively harmless form of strong religiosity or whether it is associated with hostility towards other groups is also an open question. Research on Christian fundamentalism has repeatedly demonstrated that there is a strong connection with out-group hostility, but no solid evidence is currently available that allows us to determine whether this also holds true for Muslims. On the basis of a survey among Turkish and Moroccan immigrants and their offspring as well as native comparison groups in six West European countries this paper investigates four key questions: - What is the extent of religious fundamentalism among Muslim immigrants and their offspring and how does it compare to native Christians? - What are the socio-economic determinants of religious fundamentalism among Muslims and to what extent are they similar to those among Christians? - Can religious fundamentalism among Muslims be distinguished from other indicators of religiosity, as research has found to be the case for Christian fundamentalism, or is it an inherent component of strong Islamic religiosity? - What is the relationship between religious fundamentalism and hostility towards other groups and is this relationship similar among Muslims and Christians?