Fipronil formulations and fipronil/S-methoprene co-formulations are provided herein. These formulations optionally contain one or more additional active ingredient(s). The formulations comprise an organic solvent, an alcohol co-solvent, and one or more antioxidants and without any crystallization inhibitor. The formulations provided herein are antiparasitic, and can be used, for example, to combat dog and cat parasites, such as, fleas and ticks.
