Syphacia muris parasitism was eliminated from rats and voles by feeding fenbendazole-medicated chow (150 ppm) for five 7-day periods; treatment periods were separated by 7-day periods of feeding non-medicated chow, yielding atotal treatment course of 9 weeks. No other manipulations to facilitate eradication, including the use of filter tops, autoclaved cages, environmental decontamination, colony depopulation, breeding cessation, and research restriction, were done. The examination of 3143 cellophane-tape impressions of the anus and 160 cecal examinations from euthanized rats and voles during the treatment period and for 7 months afterwards confirmed the efficacy of treatment. Treatment was rapidly effective in voles. In rats, pinworm eggs persisted at high levels for 2 weeks after the start of treatment, but no eggs were found after 22 days.