The invention provides an ectoparasiticidal formulation whichcomprises an ectoparasiticidal agent, preferably fipronil, as the pesticidally-activeingredient, together with a crystallization inhibitor selected from thegroup consisting of alkyl-substituted pyrrolidones, and optionally, one ormoreorganic solvents and/or co-solvents. Preferably, the crystallization inhibitorisN-octyl pyrrolidone, and the organic solvent, if present, preferably comprisesdiethylene glycol monoethyl ether. Theectoparasiticidal formulationsurprisingly yields a significantly enhanced speed of kill. Methods formanufacturing the formulation, as well as methods for treating animalsinfested with ectoparasites, are also disclosed.
