Within navigation pools of the upper Mississippi River, the caddisflyOecetis inconspicuauses molluscs for pupation sites. This association apparently forms out of the need ofOecetisfor a solid substrate to pupate on within a habitat dominated by soft shifting substrates.Oecetisgenerally colonized pelecypods and larger gastropods with an exposed shell surface area > 2.5 cm2.Oecetiswas found only on the posterior exposed end of clams, but occurred somewhat randomly on snails. Few pupae were found on dead molluscs.Oecetispupates on molluscs that can maintain a portion of their shell above the surface of the substrate, thus preventing pupal burial.