The present invention relates to an active composition for controlling or eradicating Phthiraptera, Siphonaptera and Acarina Pests, typically in domestic animals, comprising a synergistic combination of at least one A83543 compound and at least one macrocyclic lactone. The invention also relates to the use of the active composition in pesticidal formulations, the formulations themselves and to the various applications of those formulations as pesticides, specifically in controlling all species of Phthiraptera, Siphonaptera and Acarina pests, typically in domestic animals. Such applications include the control of such external Phthiraptera, Siphonaptera and Acarina pests in domestic animals including but not limited to sheep, cattle, poultry, pigs, goats, camelids, horses, dogs and cats.