Injectable compositions comprising antiparasitically active chitin synthesis inhibitor substances are described, for controlling parasitic infestations including infestations with sea lice and isopod species in farmed fish. The compositions are optionally formulated in a fish vaccine. The antiparasitically active chitin synthesis inhibitor substances protect the fish against parasites for a substantial period of time after injection. Useful embodiments include compositions with lufenuron that confer protection against new parasitic infestation for an extended period of time after treatment.
