Silver nanoparticles have been incorporated into a wide variety of consumer products, ideally acting as antimicrobial agents. Silver exposure has long been known to cause toxic effects to a wide variety of organisms, making large scale production of silver nanoparticles a potential hazard to environmental systems. Here we describe the first evidence that an organism may be able to sense manufactured nanoparticles in a complex, environmentally relevant exposure and that the presence of nanoparticles alters the organism's behavior. We found that earthworms (Eisenia fetida) consistently avoid soils containing silver nanoparticles and AgNO(3) at similar concentrations of Ag. However, avoidance of silver nanoparticles occurred over 48 h, while avoidance of AgNO(3) was immediate. It was determined that avoidance of silver nanoparticles could not be explained by release of silver ions or any changes in microbial communities caused by the introduction of Ag. This leads us to conclude that the earthworms were in some way sensing the presence of nanoparticles over the course of a 48 h exposure and choosing to avoid exposure to them. Our results demonstrate that nanoparticle interactions with organisms may be unpredictable and that these interactions may result in ecologically significant effects on behavior at environmentally relevant concentrations.