The virtual communities are settings for knowledge exchange, especially tacit knowledge. In large organizations, with branch offices or geographically dispersed functional units, knowledge exchange is generally restricted by the lack of convergence of people in the same physical space or schedule. In this investigation we have examined the behavior of the members of two virtual communities inside a large government institution. Both communities have several hundreds of members, but they differ in their type of organization. As a result we identified enablers and inhibitors for knowledge exchange within the virtual communities. The two main enablers are the autonomy of the members and the rewards for participation. The two main inhibitors are the heterogeneity of subjects or contents and the size of the virtual community. From these results we propose policies and procedures that counteract inhibitors and strengthen enablers for knowledge transferring.