The relative efficacy of a single 400 mg dose of albendazole or mebendazole in the treatment of nematode infections was assessed in 2- to 9-year-old children living in two different Bolivian rural communities. Both agents were equally very effective (100% cure rate) in treating ascariasis. Albendazole was clearly more active than mebendazole against hookworm infections, both in terms of egg reduction rate (92.8% vs. 62.4%) and cure rate (81.8% vs. 17.2%). As far as trichuriasis is concerned, albendazole produced a higher egg reduction rate than mebendazole (45.7% vs 15%), but a lower cure rate (33.3% vs 60%). Both drugs were well tolerated.