ABSTRACT: The amount of hydrogen peroxide (H₂O₂) used in the treatment of salmon lice in Norwegian salmon farming increased from 308 tons in 2009 to 43246 tons in 2015. For 2016 and 2017, however, the consumption was reduced to 26597 and 9277 tons, respectively. The use of this compound may have negative impacts on benthic fauna underneath the fish farms and, in particular, on polychaetes, which can be found in large numbers at the bottom under fish farms where they play a key role in the turnover of organic waste from the farm. The tolerance of Capitella sp. and Ophryotrocha spp. to a 1 h exposure to H₂O₂ (0, 100, 200, 400, 800, 1200 and 1800 mg l^-1) was evaluated. The recommended dose for treatment of the salmon is 1800 mg l^-1. Following exposures, the polychaetes were reintroduced into clean sea water. Both polychaete species experienced high cumulative mortality during a 72 h post-exposure period. The mortality showed to be dose dependent, with the highest dose giving the highest mortality. The 50% lethal concentration (LC₅₀) of Capitella sp. was significantly higher than the LC₅₀ of Ophryotrocha spp. at the same exposure time (p < 0.05). The 50% lethal time of Capitella sp. was significantly longer than that of Ophryotrocha spp. at the same concentration (p < 0.05). The results show that 1 h exposures to H₂O₂ at all the tested concentrations had irreversible negative effects on both polychaete species.