The amphiatlantic distribution of the genus Planaria is incompatible with our current hypothesis of the historical biogeography of freshwater planarians. New anatomical studies suggest the possibility that the genus is not strictly monophyletic; new karyological data are strongly corroborative of that conclusion. The karyotypes of the American species of Planaria are almost identical with those of many other North American Planariidae and quite distinct from that of the European species. These findings are inconsistent with the hypothesis that the species of Planaria form a strictly monophyletic group. Therefore the amphiatlantic disjunction is apparent, not real, and the earlier biogeographical hypothesis is protected from one potential falsifier.