Seasonal fluctuation of Beet armyworm, Spodoptem exigua, adults and larvae were momtored by pheromone trap and direct obseration in the welsh onion field, respecLively. Adult peaked on mid~late November and occurred 4 Limes a year by pheromone trap at Yesan, 1990. There were 3 peaks a year at Asan, 1991. The highest number of adults were caught on early September. In Suwon, the yearly number of adults caught by pheromone trap was high in the order of 1990, 1991, and 1992. In 1992, the moLh was initily caught on mid April by pheromone trap at Koheung, Chonnam, and it vms 3 month earlier than that at Suwon. The larvae of beet armyworm at welsh onion field at Asan, 1991 was first found on late June and gradually increased until mid September. The density at peak occurrence was about 20 individual per 100 hills of welsh onion. The peak of the larvae appeared 20 days after peak emergence of adult. It is expected that there are 4 times of occurrence when the first egg of beet armyworm IS laid on mid May at Suwon. It takes 48, 25, 23, and 58 days to complete 1st, 2nd, 3rd. and 4th generation, respectively.