There has been a lot of concern about the fate and effects of engineered (produced or manufactured) nano-particles (ENPs) due to their rapidly increasing application in different products. Metal ENPs released into the environment, intentionally and accidently, are expected to end up in the aquatic environment via sewage treatment plants, waste handling or aerial deposition. Compared to ‘common metal’, metal ENPs have high reactivity due to their small size which may have implications for bioavailability and thus toxicity in aquatic organisms. Due to the chemical and physiological properties of metals they will sorb to organic matter in the water column and subsequently accumulate in sediments. Thus, metal ENPs may pose a high risk for aquatic organisms, especially for deposit feeders. However, studies of their bioavailability in the aquatic environment are limited, and the toxic mechanisms of metal ENPs in aquatic invertebrates is not yet clear. There is a long history of using gold (Au) in pharmacy and material science. We examined the toxicity of sediment-associated Au in three different forms (nano sized, micro sized and ionic Au). The deposit feeder, Capitella teleta, was exposed to 5 concentrations of sediment-associated Au (for each Au form) during 14 days. Effects were evaluated as worm mortality and growth. In addition, concentrations of Au were measured in exposure systems after 14 days (i.e., worm tissue, sediment, fecal pellets). The following questions will be addressed: -Is Au toxicity related to form? -Does Capitella teleta affect the environmental fate of Au and does the fate depend on metal form?