Ivermectin was evaluated against natural and artificial pinworm (Syphacia) infections in mice. Ivermectin given in the diet for 6 days at 0·0005% was 99% effective against both immature and adult worms. A diet level of 0·0004% reduced immature and mature pinworm by 99 and 75%, respectively but 0·0001% was inactive. One oral dose of 2·0mg/kg was 100 and 97% effective against gravid females and immature worms, respectively. A dose of 1·0 mg/kg was 96 and 66% effective against the same parasitic stages. A similar effect was observed against adult male worms where 94 and 86% were removed by one oral dose of ivermectin at 2·0 and 1·0 mg/kg, respectively.