A combination of an anti-parasitic agent, such as azamethiphos, and fresh water for use in the treatment or prevention of a marine parasite infection in a fish, wherein the use is sequential, separate and/or simultaneous. Typically, the use is sequential, and the anti- parasitic agent is applied after an initial period in the fresh water. The treatment is suitable for a variety of fish, for example Salmo salar, sea bass, sea bream, and is effective against parasitic infections such as those mediated by Lepeophtheirus salmonis, Caligus rogercresseyi, Caligus clemensi and amoebic gill disease, The combination also provides methods of enhancing the effectiveness of an anti-marine parasitic agent, reducing the resistance of marine parasites to an anti-marine parasitic agent, treating a marine parasite infection in a fish, and preventing re-infection of a fish with a marine parasite.
