The treatment and control of opisthorchiasis relies on a single drug, praziquantel; hence, there is a need to develop novel opisthorchicidal drugs. We investigated the in vitro and in vivo activity of the antimalarial mefloquine against Opisthorchis viverrini. Hamsters infected with O. viverrini for 2 weeks (juvenile infections) and 4 weeks (adult infections) were treated orally with single 200-400-mg/kg oral mefloquine. Worm burden reductions were assessed against untreated control hamsters. Worms were incubated in the presence of 10 and 100 microg/ml mefloquine. Scanning electron microscopy was used to examine adult O. viverrini after recovery from hamsters and following in vitro incubation. A single oral dose of 300-mg/kg mefloquine resulted in worm burden reductions of 88.5% (juvenile infection) and 96.0% (adult infections), respectively. Incubation with 10 and 100 microg/ml mefloquine resulted in rapid death of O. viverrini. Extensive tegumental disruption such as blebbing, sloughing, and furrowing was seen on worms incubated in vitro and on flukes recovered 48 h posttreatment. In conclusion, we have documented promising opisthorchicidal activities in hamsters and in vitro with the tegument being an important drug target. Proof-of-concept studies with mefloquine could be considered in opisthorchiasis patients.