In the high middle ages, various kinds of populer religious movements prevailed all over Western Europe. Among them the most powerful aud determnied enemy of the catholic church was the Cathars. They practiced the severest asceticism, living as nomads, dedicated to poverty and preaching, and wholly without resources. The people were fascinated by the Cathars, and wondered if "true monks" had at last appeared to satisfy their yearnings. However, the Catharist's teaching was not truly Christian, but a melange of material taken from Gospel and dualistic beliefs which were of Manichaean and Gnostic origin. We find that the early Dominicans were incessantly occupied with fighting these heretics and arguing against them. Although this heresy flourished in Italy under the very eyes of St. Francis, it appeared that their beliefs neither acted upon nor influenced nor aroused the reactions of the saint and his followers. We did not find that he was fighting these heretics. It is true that St. Francis and his desciples did not undertake to fight the Cathars by means of polemical preaching. But this does not mean that he did not know of this heresy and its menace to the Catholic world. If we examine closely the texts of St. Francis' opuscules, we would be surprised to see that the whole religious life of the saint was quite a contrast to the Catharian teaching. His piety to the only one God, his admiration for all the created world including worldly possessions, his passionate love of the humanity of Christ, and his vivid experience of the real presence of Christ within the eucharist were silent but most powerful refutations to the dualism of the Catharists, This dualism rested upon the antagonism of two Gods, one of evil intent, the other of spiritually good. In this, we can surmise that St. Francis knew thoroughly about the'Catharist's teaching and its danger to orthodox Christianity. He undertook to overcome these difficulties by the example of his religious life itself.