Efficacy of selamectin was studied in naturally acquired S. muris infections in rats. Fourty-eight S. muris-positive rats were divided into six treated and two control groups. Selamectin (6 mg/kg) was applied topically to the skin in a single spot at the base of the neck in the treatment group. The rats of treated and control groups were necropsied on the 24th day after the treatment. Topical selamectin was found to be 40.7-63.3% effective (based on egg per gram method) in eliminating S. muris infection in rats. The efficacy of the treatment against S. muris (based on adult worm counts) in male and female rats was 35.14-58.88%, respectively (mean 48.39%).