From July 1990 to February 1991, nematode numbers in free-ranging snowshoe hares (Lepus americanus) at Kluane Lake, southwestern Yukon, Canada, were manipulated by subcutaneous injection (0.4 mg/kg) of ivermectin. Three field experiments were conducted to determine the degree of helminth loss associated with a single administration of ivermectin; the length of time that ivermectin was effective in reducing worm numbers; and the effect of repeated ivermectin administration in reducing worm numbers. Numbers of the nematodes, Protostrongylus boughtoni and Nematodirus triangularis were reduced by approximately 80% 2 wk after treatment with a single dose of ivermectin, and were still significantly lower than controls at 4 wk. However, beyond 2 wk, ivermectin did not affect the rate of acquisition of new worms of either species. All treated groups contained one or more hares in which numbers of P. boughtoni and N. triangularis were not reduced. In addition, ivermectin had no effect on numbers of Trichuris leporis or Passalurus sp. Overall, ivermectin was not as effective against the nematodes of free-ranging hares as has been reported for nematodes of domestic and laboratory animals.