The role that shared group membership plays in decisions to trust others is now well established within social psychology. A close reading of this literature, however, shows that this process is often moderated by other variables. Currently, we examined one potential moderator of this process. In particular, we evaluated the role that common knowledge of a shared social group membership between self and a to-be-trusted stranger provides as a basis for trusting this stranger. This common knowledge emerges when the truster knows the group membership of the to-be-trusted other, and believes that this other also knows the group membership of the truster. In two experiments, using pre-existing and minimal groups, we show that people are more likely to trust an in-group member over an out-group member under conditions of common group-membership knowledge rather than private group-membership knowledge (i.e. other does not know truster's group), even when they could choose not to trust anyone. The manner in which these data add to current understandings of group-based trust in strangers is discussed. Copyright © 2011 John Wiley & Sons, Ltd.