Recent waves of immigration have changed the demographic face of European societies and fueled considerable debate over the consequences of ethnic diversity for social cohesion. One prominent argument in this debate holds that individuals are less willing to extend trust and solidarity across ethnic lines, leading to lower social capital in multiethnic communities. We present a direct test of this proposition in a field experiment involving native-immigrant interactions in Zurich's Central Train Station. Our intervention consists of approaching commuters with a small request for assistance (borrowing a mobile phone), which we take as a measure of prosociality. We further differentiate between reactions towards natives as well as both high- and low-status immigrant groups. Compared to native-native interactions, we find lower solidarity in native-immigrant encounters, especially in cases involving stereotypically low-status immigrants. In exploratory analyses, we further show that discrimination only obtains in 'low cost' situations where commuters could easily justify not helping (e.g. by claiming not to carry a phone). Overall our results shed light on key theoretical mechanisms underlying patterns of solidarity in contemporary multiethnic societies.