Hydrogen peroxide (H2O2) was the only effective alternative treatment to emamectin benzoate (EMB, SLICE, Schering Plough) for the sea louse Caligus rogercresseyi in Chile during the period from February to September 2007. This introduction was due to the loss of sensitivity of lice to EMB after 7 years of exclusive use. A detailed field appraisal of treatments was carried out on salmon farms at two sites near Puerto Montt (Lat 42 degrees S) in August 2007 to evaluate the efficacy of hydrogen peroxide in the control of C. rogercresseyi. A skirt was used on one farm for treatments and an enclosed tarpaulin in the other. Hydrogen peroxide was applied by bath at a concentration of 1.5 g l(-1) for 20 min treatment but this did not kill C rogercresseyi. Most of the parasites recovered from the treatment and were available to infest new hosts. Treatments were carried out in tanks under laboratory conditions to test the findings obtained in the field and these gave similar results. Caligus recovered completely after 30 min post treatment. (C) 2010 Elsevier B.V. All rights reserved.