Criteria for distinguishing introduced from endemic peracaridan crustaceans were used to deduce that a human-borne global invasion by the Oriental isopod Synidotea laevidorsalis (Miers, 1881) has occurred in the past 100 years. These criteria concern the ecological, evolutionary, and geographical attributes of introduced species. The criteria were used first to hypothesize that Synidotea laticauda is an introduced species in the eastern Pacific that arrived on the hulls of nineteenth-century sailing ships. The hypothesis was tested by searching for previously described conspecifics throughout the world. The search culminated in discoveries that Synidotea laticauda Benedict, 1897 of the eastern Pacific and Synidotea marplatensis (Giambiagi, 1922) of the Atlantic coast of South America are misidentified populations and thus synonymies of S. laevidorsalis. Synidotea brunnea Pires and Moreira, 1975, of central Brazil is also a probable junior synonym of S. laevidorsalis. The discovery of these synonymies was thus based upon predictive criteria rather than inductive classical, taxonomic revisions. The errors in species identifications indicate that the prevalence of marine and estuarine introductions has been underestimated and that the extent of many introductions remains poorly resolved.