Paromomycin sulfate (aminosidine) at a single dose of 32 to 53 mg/kg was orally given to 24 cases with proven diphyllobothriasis. Evaluation of efficacy of the drug was based on stool examination for the eggs after 3--4 weeks of treatment. The cure rate was obtained as 96% (23/24), and 30 worms were expelled from 24 patients. Only 1 unsuccessfully treated case of 34-year-old man was retreated at the same dose of the drug 3 weeks later to obtain the cure. Thirty worms were composed of a single worm each from 21 patients, 2 worms from a patient, 3 from 1, and 4 from 1. Scolices of 7 (23.3%) out 30 worms were found. Vomiting as side effect of the drug was observed in only a case of 4-year-old girl at 40 minutes after administration of the drug but it was mild and transient. Clinical symptoms or complaints before treatment were as follows; abdominal discomfort in 12 cases, abdominal pain in 7, diarrhea in 4, fatigue in 2, tinnitus, vomiting and frequent stool in 1 each. Seven cases were almost asymptomatic. Morphological changes of the worms immersed in paromomycin solution (aminosidine) (1.66 mg/ml) for 1, 2 and 3 hours were observed in comparison with worms kept in physiological saline solution. The destructive effects were fragmentation, dissolution and desquamation of the outer cuticle and basement membrane with PAS stain at 3 hours of the experiment. The damages were also demonstrated in subcuticular tissues composing of muscle layer and parenchymal cells.