The present invention provides a method of pesticidal treatment of vertebrate aquatic animals, in particular fish. The method includes applying to said aquatic animals, typically topically or, more preferably, by administration into water in which said aquatic animals are contained, a topical pesticidal composition comprising a pyrethroid and an organophosphate biocide in a weight ratio of 1:20 to 10:1, particularly 1:10 to 5:1, especially 1:3 to 3:1.
