Is religion used as a basis for discrimination? To find out, we used a new experimental approach which measures subjects' willingness to discriminate (WTD) in an incentive compatible way. In a binary trust game more religious trustors revealed significantly higher WTDs which were related to both greater belief and investment in the trustworthiness of high-religiosity co-players. The results support a role for religion as a signal for trustworthiness. About half of the subjects discriminate, as trustor or trustee, on the basis of co-player religiosity or religious affiliation. Discriminators are willing to incur a higher cost for their discriminatory acts to be carried out. Trustors and trustees of all levels of religiosity believe more strongly in the trust and trustworthiness of higher religiosity co-players. Trustors and trustees of higher religiosity expect less trustworthiness and trust in the face of religious affiliation differences with co-players. Actions are consistent with beliefs. Trustors of higher religiosity are more trusting towards more religious trustees. Trustees are neither more nor less trustworthy towards more religious trustors. Trust and trustworthiness is not determined by differences in religious affiliation alone.