Two weeks after the unsuccessful treatment with mebendazole — 400 mg daily during four consecutive days —, 101 persons living at a semi-close community, 50.5% infected with Hymenolepis nana, mostly children from 2 to 6 years old, were treated with praziquantel, two single oral doses of 20 to 25 mg/kg administered ten days apart. The diagnosis of hymenolepiasis as well as the parasitological controls of cure performed on the 7th, 14th, 21st, 30th, 60th and 90th days after the second dose administration, were ba- sed on stool examinations according to the quantitative Kato/Katz method. The drug tolerance was excellent and the negativation of the stool examinations occurred independently from the initial worm burden. H. nana eggs were found on the 7th and 14th days post-treatment in nine and in two patients, respectively. However, these eggs were distorted. In the control made on the 21st day all results were negative indicating a 100% cure rate. From the 30th day on normal eggs were detected in three children. Considering the late positivity of their examinations and that they were the only persons living part-time outside the community, such cases were classified as reinfection. By virtue of these results it was concluded that this dose schedule, based on the experimental investigations undertook by CAMPOS et al. (1983), is safe and 100% efficacious for treating hymenolepiasis, particularly envisageing its erradication in close communities.