This article reports the results of an online experiment conducted in two ethnically fragmented societies in Spain: the Basque Country and Catalonia. It tests the effect of co-ethnicity on trust and reciprocity. Ethnicity was manipulated in the experimental context using three ethnic attributes: ancestors' origin, language and name. Additionally, the article reports a comparison of general levels of trust in the two regions. No co-ethnicity effect on trust is found, but there is significantly more reciprocal behaviour between Catalan speakers in Catalonia. Higher levels of trust and reciprocity are found in the more homogeneous society of the Basque Country. The lack of co-ethnicity effect on trust is especially significant given that the Basque Country has experienced decades of terrorism along ethnic lines.