115 PSICOLOGIA SOCIALE n. 1, gennaio-aprile 2019 With the aim of understanding the relationship between the presence of immigrants and preju- dice towards them, we tested two competing predictions drawn from the outgroup threat theory and the contact hypothesis. We also tested whether such relationship differed depending on individual political ideology. The results of a multilevel analysis conducted on longitudinal data from an Italian sample (N = 3,871, nested in 103 Italian counties) revealed that, in line with the group threat theory, the association between the immigrants’ group size and prejudice was positive only among right-wingers. Strengths, limitations, and possible developments of this research are discussed. Political ideology moderates the relationship between outgroup size and prejudice toward immigrants A longitudinal, multilevel study Silvia Russo, Daniela Barni, Nicoletta Cavazza, Michele Roccato and Alessio Vieno In a time of increasing immigration flows, understanding how citizens’ attitudes towards the newcom- ers evolve becomes of paramount importance. In Europe, immigra- tion flows seem to go along with a rise in anti-immigrant sentiments (Semyonov, Raijman & Gorodzeisky, 2006). However, the relationship between them is not clear yet. The present paper aims at contributing to this understanding by analysing the link between the outgroup size and prejudice towards immigrants as a function of individual differ- ences (i.e., political ideology). 1. Outgroup size and prejudice Two main perspectives have been used to explain the relationship between out- group size and anti-outgroup sentiments. The first one is the group threat theory. It posits that a large outgroup size fosters the perception of threatened group in- terests, resulting in more negative attitudes toward the outgroup (Blalock, 1967; Blumer, 1958). According to this perspective, when the size of an outgroup is large, hostile intergroup attitudes are likely to rise. Blalock (1967) points out two reasons