A variety of competitive advantages accompany colonization of hard substrates in aquatic settings, and as a result, many organisms adopt this mode of life. Similarly, adverse effects may occur to organisms that have themselves been encrusted. Many marine basibiont faunas (organisms that provide a substrate for populations of attached epibiontic organisms), including bivalve mollusks, are known to employ a range of chemical and physical deterrents to minimize the incidence of epibiotic attachment, with its attendant detrimental impacts. Freshwater basibiont and epibiont associations, however, are poorly documented. We collected an assemblage of dead, empty freshwater mussel (unionid bivalve) shells from the Saint John River, New Brunswick, Canada. Some unionid specimens exhibited colonization by attached Trichoptera (caddisfly) cases, in either their final instar larval or pupal developmental stages. The trichopteran assemblage, identified from case morphology, included abundant Neophylax sp. together with common Goera sp. and rarer Helicopsyche borealis. For all genera, site-selective attachment occurred upon the shell surfaces. Crystalline aragonite and organic layers exposed within taphonomically damaged regions of molluscan shell were not colonized. Attachment was instead confined to areas of intact periostracum posterior to the umbonal region of the shell, the periostracum perhaps being selected for its rough surface texture in comparison to aragonitic layers. Trichoptera cases dislodged during handling of specimens revealed no damage to the underlying shell. Observations undertaken on the bivalve shells using scanning electron microscopy suggest that physical barriers to attachment are absent within these faunas, and evidently, any chemical barriers, if present, are largely ineffective.