The in vitro activity of root-tuber-peel extract of Flemingia vestita, an indigenous plant consumed by the natives in Northeast India, was tested against helminth parasites. Live parasites (nematode: Ascaris suum from pigs, A. lumbricoides from humans, Ascaridia galli and Heterakis gallinarum from domestic fowl; cestode: Raillietina echinobothrida from domestic fowl; trematode: Paramphistomum sp. from cattle) were collected in 0.9 % physiological buffered saline (PBS) and maintained at 37 +/- 1 degrees C. In vitro treatment of the parasites with the crude extract (50 mg/ml) in PBS revealed complete immobilization of the trematode and cestode in about 43 and 20 min, respectively. However, the cuticle-covered nematodes did not show any change in physical activity and remained viable even after a long period of exposure to the extract. Exposure of R. echinobothrida to genistein (0.5 mg/ml), an active principle isolated from the root-tuber peel, caused spontaneous loss of movement (paralysis) in 4.5 h, which was slower than the time required for praziquantel, the reference flukicide and cestodicide. The treated parasites showed structural alteration in their tegumental architecture. This study suggests the vermifugal activity of this plant extract against trematodes and cestodes.