Giardiasis and soil-transmitted helminthosis are prevalent in many parts of the world. Application of a single drug for treatment of both giardiasis and soil-transmitted helminthosis is highly desirable. One hundred children of both sexes, ranging from 7 to 12 years old with Giardia intestinalis cysts or trophozoites in their stool samples, were randomly separated into two groups of 50 individuals each. Each group received either mebendazole tablets 200 mg thrice daily for 5 days or metronidazole tablet 15 mg/kg/day in three divided doses, for 7 days. The results were evaluated by microscopic examination of stools on two successive occasions: 1 week and 2 weeks after treatment. Each occasion consisted of three stool examination on 3 successive days. The cure rate of mebendazole-treated children was 43 out of 50 (86 per cent), and for metronidazole it was 45 out of 50 (90 per cent) with no statistical difference between the two groups. No side-effects were observed in the mebendazole-treated group, whereas nausea, anorexia and metallic taste were observed in 4.9, 6 and 24 per cent of metronidazole-treated children, respectively. In conclusion, the use of mebendazole, which is less toxic with no side-effects, is available and effective on Giardia intestinalis as well as intestinal nematodes, is recommended especially in the areas where giardiasis and intestinal nematodiasis are prevalent and overlap.