By analysis of feces collected from bats in the field, we determined that aquatic insects, particularly chironomid Diptera, made up the major portion of the diet of Myotis lucifugus at sites in southern Ontario, northern New York, and Nova Scotia. The diets of adult males reflected the available insect prey as sampled by a malaise trap, while those of lactating females included proportionally more caddis flies and moths than were present in the malaise trap samples. The diets of subadults of both sexes showed greater variation than those of adults, although chironomids and caddis flies were important components. While we observed the aforementioned pattern at sites in Nova Scotia, northern New York, and southern Ontario, the diets of adult M. lucifugus in northern Ontario were as variable as those of subadults from more southerly areas. We suggest that M. lucifugus is opportunistic in its feeding habits, and that the adults efficiently harvest s warms of aquatic insects, a trait not fully acquired by the young we sampled at the end of August.