The study was carried out from September to November 1997 in Phrae Province of northern Thailand. A total of 95 adult patients with Opisthorchis-like ova in their stools were randomly treated with two different manufactured Praziquantels. Group 1, consisting of 49 patients, received a single dose of 40 mg per kg Praziquantel manufactured by the Thai Government Pharmaceutical Organization. Group 2 (46 patients) received Biltricide at the same dosage. Haplorchis taichui, H. yokogawai, Echinostome spp., O. viverrini, Taenia saginata and Enterobius vermicularis were expelled in the stools after treatment. Minute intestinal flukes were detected in 64% of patients. O. viverrini was found in lower proportion of 17%. By formalin-ether concentration examination one stool specimen from each patient, the cure rate in both groups on the 30th day of treatment was 100%. The side effects of the two different Praziquantel treatments were mild with no significant difference. Praziquantel, regardless of its manufacture, proved effective against O. viverrini and other minute intestinal flukes (H. taichui, H. yokogawai and Echinostome spp).