Whole‐animal anatomy is used in a cladistic analysis of five genera of brackish‐water hydrobiids living in the western Atlantic.Antillobia margalefi gen.&sp. nov.(Hydrobiidae: Littoridininae) is described based on specimens from Lago Enriquillo, an inland saline lake in the Dominican Republic. The topology (Hydrobia(Spurwinkia(Heleobia(Heleobops(Antillobia))))) is considerably better than any alternative. Among 27 characters and a total of 39 character states used in the analysis, only two convergences are detected, and these involve the genitalia. Detailed anatomical data are shown to be necessary to assess relationships in the presence of constraints on morphological evolution imposed by a very small s