Composition which is useful in particular for the treatment and protection of domestic animals which are infested with parasites or are likely to be infested with them, these compositions comprising, in the form of a ready-to-use solution:a) an insecticidal active substance of formula (I),b) a crystallization inhibitor,c) an organic solvent having a dielectric constant of between 10 and 35, preferably of between 20 and 30,d) an organic co-solvent having a boiling point below 100° C., preferably below 80° C., and a dielectric constant of between 10 and 40, preferably of between 20 and 30.
