We use carbon and nitrogen isotope data to elucidate the diets of larval and adult Brychius hungerfordi Spangler in streams in Northern Lower Michigan. Diet analysis suggests that feeding behavior during the larval life stage is different from that of the adult. Larvae appear to prefer the alga Dichotomosiphon tuberosus (Braun) Ernst. Adults appear to be generalists, feeding on Dichotomosiphon, Cladophora Kuetzing and Chara Linnaeus as well as the epiphytic diatom Cocconeis Ehrenberg. Dichotomosiphon, although widespread, is not common. Its presence may be an important factor in determining the distribution of the Federally endangered beetle.