Insect pests of eggplant, pepper, Chinese cabbage, watermelon, cucumber, melon, and tomato seedlings were surveyed in eleven propagation houses from 1998 to 1999. Ten species of insect pests of nine genera in eight families, two species of mites of one genus in one family, and three species of slugsor snails in three genera belonging to two families were found from the observed seedlings but Bradysia agrestis was the most serious pest out of them. Thus, occurrence and damage of B. agrestis were investigated in two propagation houses all the year round. In the observation of seedling mortalities of seven crops in May, July, October and the following February, only watermelon seedlings were withered and dead by the larva of B. agrestis with different mortality depending on time representing 81.9%, 41.3%, 54.9%, and 79.1%, respectively. All the developmental stages of B. agrestis were found every month throughout the year. Larval density and adult density were different according to season with the highest numbers in May showing 34.9 and 407.4, respectively. Mortality of watermelon seedlings was higher in April and May than the other months showing 83.3% in April and 82.4% in May. The number of adults attracted to sticky trap was also different depending on card colour. The number of adults was much higher on yellow sticky card (326.2) than blue sticky card (20.2). In the investigation of the number of adults on yellow sticky card at 25 cm and 120 cm above the bench and 10 cm under the bench, more numbers of adults were caught at 25 cm above the bench (273.7) and 10 cm under the bench (320.1) than 120 cm above the bench (27.9). Mortalities of cucumber, pepper, and watermelon seedlings after transplanting in greenhouses were not significantly different depending on culture method.