Based on a series of ethnographical observations carried out in a French immigration detention center where undocumented foreigners await deportation, this contribution describes the work of lawyers from the French human rights organization CIMADE who daily provide detained foreigners with legal counsel. The article first shows how these lawyers master immigration law, and use it as a strategic resource in order to legally challenge the detainees’ deportation orders. It then draws attention to the contribution of this legal activism to the differential management of illegal immigration. Each intervention of the CIMADE lawyers indeed supposes re-examining the judicial and physical “border” separating deportable foreigners from those who may be legalized, but it re-produces and re-enacts it at the same time.