As the application of nanomaterials to science and technology grows, the need to understand any ecotoxicological effects becomes increasingly important. Recent studies on a few species of fishes and invertebrates have provided data which suggest that harmful effects are possible. The way in which nanoparticles are taken up by aquatic organisms, however, has been little studied. We examined uptake of nanoparticles by two species of suspension-feeding bivalves (mussels, Mytilus edulis; oysters, Crassostrea virginica), which capture individual particles < 1 microm with a retention efficiency of <15%. Given this limitation, it would appear that nanoparticles could not be ingested in large numbers. During certain times of the year, however, >70% of suspended particles are incorporated within aggregates that are > 100 microm in size. Therefore, we delivered bivalves fluorescently labeled, 100-nm polystyrene beads that were either (1) dispersed or (2) embedded within aggregates generated in the laboratory. Results indicate that aggregates significantly enhance the uptake of 100-nm particles. Nanoparticles had a longer gut retention time than 10-microm polystyrene beads suggesting that nanoparticles were transported to the digestive gland. Our data suggest a mechanism for significant nanoparticle ingestion, and have implications for toxicological effects and transfer of nanomaterials to higher trophic levels.