The subject matter disclosed herein is directed to stable, highly-effective topical formulations comprising permethrin, fipronil and a solvent system that is sufficient to solubilize these two active ingredients and limit degradation of fipronil to its sulfone, and their uses in topical applications on animals and the environment. Useful formulations comprise from about 30% to about 55% (w/w) permethrin and about 2 to 15% (w/w) fipronil and a solvent system that comprises N-methylpyrrolidone and...