We have documented a large population of a rarely collected northern caddisfly, Phanocelia canadensis (Banks) (Trichoptera: Limnephilidae), on Cape Cod, Massachusetts. This species' range is generally considered to be centered in northern Canada, although there are individual records of adults from eastern Maine and New Hampshire, and a single adult was collected from Sherborn, Massachusetts, in the 1920s. The scarcity of this species in collections may reflect true rarity, with populations sparsely but widely distributed across the northern part of the continent. Alternately, it may reflect a sampling bias, in that the larvae of this species were unknown until the late 1980s, adults are diurnal and fly late in the fall, and larvae occur in wetlands and are closely associated with Sphagnum, from which they make their cases. We have carried out an in-depth habitat comparison of sites in which we found or did not find larvae on Cape Cod in seven years of intensive sampling. Habitat characteristics distinguishing wetlands with and without Phanocelia include dominance by Sphagnum sp., shrub cover, and low pH. We are currently searching in similar habitats across the state for additional populations.