The chromosomes of sea-run Atlantic Salmon from three North American rivers were studied with cell culture preparations. Extensive data from Machias River (Maine) specimens showed a sharp mode of 54 chromosomes. Modes of 55 for the Narraguagus River (Maine) and 56 for the Miramichi River (New Brunswick) represented much smaller percentages of their counts. A relatively constant arm number of 72 indicates a Robertsonian pattern of polymorphism.