PROBLEM TO BE SOLVED: To provide compositions to control and eliminate parasites living on pets, which, when administered locally onto a body, diffuse over the entire body and then dry, without any phenomenon of crystallization, without influence on appearance of the pelage and without making the pelage sticky.SOLUTION: A composition comprises: a) an insecticidal active substance of the formula (I), where R1-4 are an alkyl radical or the like, y is a halogen or the like, and p is an integer between 1 and 5； b) a crystallization inhibitor characterized by leaving only 10 or fewer crystals observable on a glass slide with a naked eye after 0.3 ml of a solution A comprising 10% (W/V) of the compound of the formula (I) and 10% of this crystallization inhibitor added to a solvent defined in c) is placed on the glass slide and left at 20°C for 24 hours； c) an organic solvent having a dielectric constant between 10 and 35； and d) an organic co-solvent having a boiling point at or below 100°C and a dielectric constant between 10 and 40 and acting as a drying promoter.【課題】体の一部に投与するだけで全体に拡散し、乾燥し、結晶化現象が起きず、毛皮の外観に無影響でべとつかない、ペットに寄生する寄生虫を抑制・駆除する組成物の提供。【解決手段】a)殺虫活性物質：(式I：R1-4はアルキル基等、ｙはハロゲン等、ｐは１-５の整数)、b)結晶化阻害剤：c)で定義される溶媒中に式(I)の化合物を10％(W/V)、この結晶化阻害剤を10％添加した溶液Ａの0.3mlをガラススライドに付け、20℃で24時間放置した後にガラススライド上を肉眼で観察した時に、観察可能な結晶の数が10個以下）、c)誘電率が10～35の有機溶媒、d)沸点が100℃以下で誘電率が10～40の乾燥促進剤となる有機共溶媒。【選択図】なし
