Nine vital tapeworms were expelled from 7 patients with diphyllobothriasis latum by the modified Damaso De Rivas' method. Seven worms of them placed in 1% and 5% paromomycin solution (37 degrees C, pH 6.8--6.9) and 2 were in physiological saline at 37 degrees C as controls. In both 1% and 5% solution, the movement of worms stopped completely within 15 minutes. When they were taken out from the solution and washed thoroughly with physiological saline solution, they did not move any longer and died soon. As for the controls, they were moving with animation in physiological saline at 37 degrees C even after 60 minutes. Pathohistologically (H.E. stain), the scolex was seriously affected down to the immature proglottid; degeneration and desquamation of the villi and cuticle with somatic cells exposed and edematous swelling of the whole body. Somatic cells had degenerative necrosis, plasmatomy and disorder in the disposition of the circular muscle and longitudinal muscle were marked. Scanning electronmicroscopy revealed that the worms treated with paromomycin had the scolex remarkably impaired and destroyed the villous epithelial layer and cuticle of the immature proglottid adjacent to the scolex desquamated exposing somatic cells regardless of the duration of exposure to and the concentration of paromomycin, while in the controls, the scolex was normal and the worm surface was closely covered with normal villi. Aminoglycoside antibiotics are known to have neurotoxic effect in general. It seems to be related to this neurotoxicity that paromomycin solution stopped the movement of worms. The changes occurring in the tapeworms were irreversible and the worms died soon. It is suggested that when patients with diphyllobothriasis latum are administered with paromomycin, the alterations of the worms induced by paromomycin are accelerated by potent human digestive juice.