Colonization of organic limnephilid, (Limnephilinae, Pycnopsyche sp. caddisfly larvae cases by bacteria was examined in a coastal plain stream in South Carolina [USA]. Bacterial densities were higher on cases than on leaves in packs in any phase of decomposition. The presence of the caddisfly is thought to facilitate the bacterial density. Microbial decomposition of cases may be an important factor in the selection of case material and in the switching from organic to inorganic materials.