Treatment of Strongyloides stercoralis infection was investigated in 2 dogs with naturally acquired, chronic-active infections, and in 3 dogs with corticosteroid-enhanced, experimentally induced hyperinfections. A single oral dose of ivermectin was given to naturally infected (200 micrograms/kg of body weight) and experimentally infected (800 micrograms/kg) dogs. Five dogs with experimental hyperinfections served as controls. Dogs with naturally acquired infections ceased to shed first-stage larvae in the feces 1 week after treatment, but 1 dog had recrudescence and required a second dose. Ivermectin was 100% effective in removing adult S stercoralis from the intestinal tract of the experimentally infected dogs, but it was not effective in removing third-stage larvae from parenteral sites. Ivermectin-treated dogs had few intestinal parasites of any stage, whereas at necropsy, 4 of 5 experimentally infected dogs not treated had massive infections (greater than 100,000 adults, greater than 92,000 larvae) in the intestinal tract, and 3 of 5 had larvae (greater than 2,500) in parenteral sites.