A spot-on formulation for the treatment or prophylaxis of parasite infestation in mammals or birds which comprises a composition comprising an effective amount of 1-[2,6-Cl 2 -4-CF 3 phenyl]-3-CN-4-[SO-CF 3]-5-NH 2 pyrazole (fipronil); and an effective amount of emamectin or a salt thereof; a pharmaceutically or veterinary acceptable liquid carrier vehicle wherein the liquid carrier vehicle comprises a solvent and a cosolvent wherein the solvent is selected from the group consisting of acetone, acetonitrile, benzyl alcohol, butyl diglycol, dimethylacetamide, dimethylformamide, dipropylene glycol n-butyl ether, ethanol, isopropanol, methanol, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, monomethylaceamide, dipropylene glycol monomethyl ether, liquid polyoxyethylene glycols, propylene glycol, 2-pyrrolidone, diethylene glycol monoethyl ether, ethylene glycol, diethyl phthalate, and a mixture of at least two of these solvents and the cosolvent is selected from the group consisting of absolute ethanol, isopropanol or methanol; a crystallization inhibitor, wherein the crystallization inhibitor is selected from the group consisting of an anionic surfactant, a cationic surfactant, a non-ionic surfactant, an amine salt, an amphoteric surfactant, polyvinylpyrrolidone, polyvinyl alcohols, copolymers of vinyl acetate and vinylpyrrolidone, polyethylene glycols, benzyl alcohol, mannitol, glycerol, sorbitol, polyoxyethylenated sorbitan esters; lecithin, sodium carboxymethylcellulose, and acrylic derivatives, or a mixture of these crystallization inhibitors. a composition comprising an effective amount of 1-[2,6-Cl 2 -4-CF 3 phenyl]-3-CN-4-[SO-CF 3]-5-NH 2 pyrazole (fipronil); and an effective amount of emamectin or a salt thereof; an effective amount of 1-[2,6-Cl 2 -4-CF 3 phenyl]-3-CN-4-[SO-CF 3]-5-NH 2 pyrazole (fipronil); and an effective amount of emamectin or a salt thereof; a pharmaceutically or veterinary acceptable liquid carrier vehicle wherein the liquid carrier vehicle comprises a solvent and a cosolvent wherein the solvent is selected from the group consisting of acetone, acetonitrile, benzyl alcohol, butyl diglycol, dimethylacetamide, dimethylformamide, dipropylene glycol n-butyl ether, ethanol, isopropanol, methanol, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, monomethylaceamide, dipropylene glycol monomethyl ether, liquid polyoxyethylene glycols, propylene glycol, 2-pyrrolidone, diethylene glycol monoethyl ether, ethylene glycol, diethyl phthalate, and a mixture of at least two of these solvents and the cosolvent is selected from the group consisting of absolute ethanol, isopropanol or methanol; a crystallization inhibitor, wherein the crystallization inhibitor is selected from the group consisting of an anionic surfactant, a cationic surfactant, a non-ionic surfactant, an amine salt, an amphoteric surfactant, polyvinylpyrrolidone, polyvinyl alcohols, copolymers of vinyl acetate and vinylpyrrolidone, polyethylene glycols, benzyl alcohol, mannitol, glycerol, sorbitol, polyoxyethylenated sorbitan esters; lecithin, sodium carboxymethylcellulose, and acrylic derivatives, or a mixture of these crystallization inhibitors.