The present invention concerns a stable aqueous nanosuspension dosage formulation for injection to fish, comprising (i) an active ingredient which is lufenuron or hexaflumuron, each in form of particles having an effective average particle size of ≦1000 nm (ii) one or more surface stabilizers selected from the group consisting of a surfactant and a polymer or a mixture thereof and (iii) water. The nanosuspension formulations of the present invention are especially suited for the treatment of salmon and provide prolonged effective protection against sea lice at sea.
