Due to wide spread applications of nanoparticles, including Ag, in our daily life, these novel particles are receiving increasing attention by risk assessors. Many studies have been performed to test the toxicity of nanoparticles at the subcellular level. Despite these findings, there is still limited information at the whole organism level as to whether metallo-nanoparticles differ in toxicity from the same metals added to the environment in ionic or bulk form. In the present study, two organisms (i.e., a marine polychaete, Capitella teleta and a marine bivalve, Macoma balthica) were exposed to sediment amended with Ag in various forms (ionic Ag(I), nano-sized Ag and micron-sized Ag), and toxic endpoints were assessed at the whole organism level. After exposure for several weeks, no significant toxic effects were detected (i.e., mortality, growth rate and condition index) at the whole organism level, although all forms of metallic Ag were accumulated in our test organisms. Our results thus did not confirm findings from studies performed at the subcellular level that have found the toxicity of Ag nanoparticles was more toxic than the corresponding metallic Ag in ionic and in bulk forms . However, more studies should be performed at the whole organism level to confirm whether the toxicity of nanoparticles can be extrapolated from that of the corresponding metallic ions which have been well studied.