A 1-year-old male dog was diagnosed with a biting louse infestation caused by Heterodoxus spiniger and was treated successfully with a single 2.5 mg/kg body weight oral dose of sarolaner. After treatment, there was a rapid improvement of the skin lesions and pruritus. The dermatological examination was within normal limits and lice were absent on day 28. To the authors' knowledge, this is the first published report of the use of an isoxazoline to treat a biting louse infestation caused by H. spiniger in a dog.