Five different dose groups were formed to evaluate the efficacy of emamectin benzoate as a treatment for Lernanthropus kroyeri (van Beneden, 1851) infestation in cultured sea bass Dicentrarchus labrax (L.). Emannectin benzoate was administered in-feed at doses of 0 (control), 10, 25, 50, and 100 mu g kg(-1) biomass day(-1) for 7 consecutive days. Parasites were counted on days 7, 14, and 21, and comparisons were made to untreated control fish. Seawater temperature was 16-16.5 degrees C. Treatment with emamectin benzoate was effective against larval and mature stages of L. kroyeri. The number of L. kroyeri in the control fish increased by 8% during the 21 days of the study, whereas their number decreased in the treated fish by 55%, 50%, 43%, and 74% at the dose of 10, 25, 50, and 100 mu g kg(-1), respectively. No differences were observed with respect to growth between the treatment groups and the control group, and no mortality was observed at the end of the study.