Several crab species are important fishery resources in Brazil and their overexploitation has lead to severe population declines in some regions. The management of populations of these species should take into account the degree of gene flow among populations in different estuaries. The goal of the present study is to assess the degree of geographical structure in the genetic diversity of the blue land crab, Cardisoma guanhumi, along the Brazilian coast. A fragment of the control region of the mtDNA (750 bp) was sequenced for 95 specimens collected across 5 Brazilian states. Analyses using F-statistics failed to indicate any evidence of geographical structure, a result that was corroborated by a nested clade analysis of the same dataset. Mismatch distribution analyses indicated that populations of the blue land crab have experienced an expansion during their recent evolutionary past. The obtained results are similar to those recently described for another sympatric crab, Ucides cordatus, particularly with respect to the extensive degree of gene flow. However, populations of C. guanhumi seem to be older than those of U. cordatus and do not show the north–south expansion found in that species.