While there is considerable information on freshwater toxicity of Ag to fish, little is known about the toxicity of Ag in marine systems. In the present study, standard 96-h acute toxicity tests were performed on seawater-acclimated rainbow trout in water at 25‰salinity. Toxicity tests were repeated in various salinities (15, 20, 25, and 30‰) at the 50% lethal concentration (LC50) for Ag in 25‰ to investigate the influence of salinity on toxicity. Computer speciation modeling by MINEQL+ was performed in order to gain insight into the variation of Ag species over a range of salinities from freshwater to oceanic seawater at S2–-free conditions. At 25‰salinity the LC50 was determined to be 3.75 μM (401 μg/L) Ag. No mortality was observed at this Ag concentration in 15 or 20‰ salinities. Mortality increased to 50% in 25‰ seawater and again to 67% at 30‰. Based on the results of speciation modeling, an increase in a more toxic AgCln species is unlikely. The increased toxicity with salinity may instead be linked to an incomplete hypoosmoregulatory ability of the rainbow trout.