There is increasing concern about the toxicities and potential risks, both still poorly understood, of silver nanoparticles for the aquatic environment after their eventual release. In this study, the toxicities of nano (< 100 nm)-, micron (2-3.5 µm)- and ionic (AgNO3)-Ag on the sediment-dwelling polychaete, Nereis diversicolor, were compared after 10 d of sediment exposure, using growth, DNA damage (comet assay) and bioaccumulation as endpoints. The nominal concentrations used in all exposure scenarios were 0, 1, 5, 10, 25, 50 µg Ag/g dry weight (dw) sediment. Our results show that Ag is able to cause DNA damage in Nereis coelomocytes and that this effect is both concentration- and Ag form-related. There were significantly greater genotoxity (higher tail moment and tail DNA intensities) at 25 and 50 µg/g dw in nano- and micron-Ag treated groups and at 50 µg/g dw in ionic-Ag treated group compared to the controls (0 µg/g dw). Nano-Ag has the greatest genotoxic effect of the three tested Ag forms and ionic-Ag is the least genotoxic, indicating different mechanisms are possibly involved to cause DNA damage by the different Ag forms. N. diversicolor did accumulate sediment-associated Ag from all three forms. Significant increases in Ag body burdens in worms treated with ionic-Ag (5, 10, 25, 50 µg/g dw), nano- and micron-Ag (25, 50 µg/g dw) were observed compared to the controls. Ag body burdens at the highest exposure concentration were 9.86±4.94, 8.56±6.63 and 6.92±5.86 µg/g dw for ionic-, nano- and micron-Ag treated worms, respectively. However, there are no form- or size-related differences in bioaccumulation of Ag. No measurable change in wet weight for any of the worm groups during the 10 d experiment was observed, indicating that this measure of growth is not a useful endpoint for detecting effects of such short duration exposures for this species.