The success rates and adverse effects of a selamectin spot-on preparation, given orally weekly or every other week, against canine generalised demodicosis were evaluated in 44 dogs, 39 with juvenile-onset demodicosis and five with adult-onset demodicosis. The dogs received selamectin at a dose of 24 to 48 mg/kg once a week, or the same dose every two weeks. Thirty-eight dogs completed the study, four dogs were lost to follow-up and two dogs died. Nine dogs went into remission: all had the juvenile-onset form of the disease. There was no difference between the two treatment groups, but dogs with clinical scores of 65 or less responded significantly better than those with a score of over 65 (P=0.0015). The most frequent adverse effects were vomiting and diarrhoea. Two dogs exhibited mild reversible neurological side effects, which resolved with cessation of treatment. Difficulties in oral administration were observed in several dogs.