A collar or other external device for pets, particularly cats and dogs, consisting of a matrix incorporating 0.1-40 wt%, preferably 1-15 wt%, based on the weight of the collar, of a substance that is effective against ectoparasites such as fleas and ticks and consists of at least one compound of formula (I). Said collar or other external device is designed to be effective for over six months against fleas and over three months against ticks. Efficacy is even maintained for several weeks after the collar or other external device is removed or lost, or when the release rate of the compound varies.
