An avermectin-based topical formulation is disclosed which is useful for prevention and treatment of head lice (Pediculus humanus capitis). This topical formulation may be formulated as a shampoo-condition which comprises an effective amount of avermectin, solubilizers, suspending agents, preservatives, non-ionic surfactants, humectants, a silicone compound, and water. Also disclosed are methods of using the topical formulations disclosed within this specification to treat either a susceptible or treatment-resistant strain of lice, as well as uses in the manufacture of a medicament for treating or preventing a lice infestation from a susceptible or treatment-resistant strain in a human patient.