The autumn migration of Mythimna separata (Walker) (Lepidoptera: Noctuidae) across the Bohai Sea was observed with a scanning entomological radar and a searchlight trap at Beihuang, an island located in the center of the Bohai Gulf of northern China, in 2003-2006. During the autumn migration, M. separata flew at the altitudes of 50-500 m, with a displacement speed of 4-12 m/s, toward the southwest. Variations of area density of the radar targets and of catches in the searchlight trap through the night indicated that the flight duration of M. separata was approximately 10 h. Based on these observations, M. separata that originated in northeastern China (i.e., Liaoning, Jilin, and Heilongjiang provinces and part of the Inner Mongolia autonomous region) could immigrate into eastcentral China and subsequently to southern China (i.e., Fujian, Guangdong, and Guangxi provinces) within a week for overwintering.