Abstract: The results of the present experiment concerning the development of Paragonimus westermani in rats and its treatment with pyquiton were as follows.1. A wandering, non-encysted adult worm and many eggs of P. westermani were recovered in the liver of a rat (No. 6) 200 days after the infection with 100 encysted metacercaria. Two adults were found in the thoracic cavity of a rat (No. 8) 230 days after the infection also with 100 cysts. However, about 99% of the young worms (adolescents) were recovered in the muscles and the remainder 1% in the other parts (liver, thoracic cavity, etc.).2. Most of the young worms were killed in six infected rats treated with pyquiton in dosage of 120mg/kg body weight/day for two days. It is suggested that pyquiton is effective in treating rats infected with the adolescents of P. westermani.