This article examines whether the support for Eurosceptic challenger parties influences mainstream party position change on European integration in Western Europe. The key finding is that Eurosceptic challenger support is capable of influencing mainstream position shifts on European integration provided that, on average, EU issues are regarded as important by the Eurosceptic challengers. Moreover, the centre-left is more affected by Eurosceptic contagion since it is influenced by both radical right and radical left Eurosceptic success, whereas the centre-right is only susceptible to radical right success. The empirical analyses are based on panel regression analysis employing expert survey data provided by Chapel Hill Expert Survey. The findings presented in this article have important implications for the study of party positioning on European integration as well as for the study of party competition in general.