The invention provides for spot-on formulations comprising fipronil from greater than 10% to 25% w/v； optionally an insect growth regulator (IGR) active agent； a crystallization inhibitor； optionally an organic co-solvent； an organic solvent； optionally an antioxidant, wherein composition results in none or minimal crystallization at the site of application over a time period over 0 to 24 hours after application to the animal. The formulations may be used for the treatment or prophylaxis of parasites of mammals and birds, and in particular, cats, dogs, horses, chickens, sheep and cattle with the aim of ridding these animals of all the parasites commonly encountered by birds and mammals. The invention also provides for effective and lasting destruction of ectoparasites, such as fleas, ticks, itch mites and lice, and of endoparasites, nematodes, such as filariae, and roundworms of the digestive tract of animals and humans.
