a r t i c l e i n f o This study evaluated the effects of eugenol on the control of monogenean infestation in tambaqui gills. Naturally infected fish were subjected to eugenol baths at different concentrations (0, 5, 10 and 15 mg L −1). After being immersed in a eugenol bath for 15 min, the fish were transferred to eugenol-free water for 30 min to remove parasites and sample for gill removal and parasite count (3 replicates per treatment, with 5 fish each). Fish sam-pling for parasite count was repeated after one week. The procedures were repeated in two other experiments, except for the bath periods, which lasted 30 min and 60 min. The bath with 10 mg L −1 eugenol solution for 60 min was the most efficient against monogeans, decreasing their count by 81%. In conclusion, eugenol is effi-cient for controlling monogenean gill infection in tambaquis. However, other studies under field conditions are needed to improve the use of this natural substance for disease control and prevention.